cross package maker: update filters

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Update filters are simple post-processing hooks. They're run after any input files have been collected, and before the target package is built.

`-u man`	Compresses man pages.
`-u appdata`	Creates `pkg.appdata.xml` for distro appcenters.
`-u desktop`	Adds a stub `fpm:pkg.desktop` file.
`-u composer`	Update a `composer.json` for `phar` target.
`-u fixperms`	Limit file permissions (`umask 755`).
`-u strip`	Strip debugging symbols from binaries.
`-u deps`	Resolve library/dependency names across distros.
`-u unprefix`	Converting system package filenames to local paths.
`-u lcase`	Lowercase filenames and directories.
`-u preprocess`	Expand preprocessing macros in source files.
`-u packfile`	Customize via external/packaged-in `Packfile` make script.

You'd usually run them with:

xpm  -s dir  -u man,appdata,desktop  -t deb  ...

They're intended for simple tasks.

Can supplant build/make steps that rather belong into the packaging process.
They're partly resembling debian-helpers.
Can either be specified as comma-separated lists, or with multiple -u or --filter flags.
Some plugins take options separated by = from plugin name, as in -u deps=fedora or -u lcase=*:php:rb

Note that filters are applied just once on the input, not on each output package in a multi-target build.

-u man

The "man" filter simply compresses manpages found in the staging path.
- It looks for a man/ directory.
- Searches for files ending in .1 or .2 or other numeric suffixes.
It's intended to avoid that step in Makefiles. (As this is clearly a packaging thing.)
-u appdata

The "appdata" filter crafts a plain PKG.appdata.xml file. Those are used by the application managers/centers in most distributions. It's an agreed format between DEB and RPM-based distros, Listaller, etc.

See http://www.freedesktop.org/wiki/Distributions/AppStream/

A few things missing here:
- We likely need a --screenshot flag. Those are the primary purpose of AppData/AppStream.
- This filter will overwrite existing appdata.xml files. So don't use it, if you're already providing one.
-u desktop

State: very crude.

With -u desktop an application icon file appname.desktop will be generated.
- Don't use this update filter if you're already providing one.
- It will be prefixed with fpm:appname to avoid collisions.
-u composer

When using the -t phar module, you may wish to combine it with an implicit -u composer update.
- Writes current version etc. to composer.json
- Creates a stub composer.json otherwise, containing:
  - name
  - description
  - license
  - homepage
  - type: Library
  - extra: [ Maintainer, Epoch, Releases (stub) ]
  - autoload (shared.phar)
- No dependency translation is handled whatsoever currently. (This plugin is primarily meant for -s src to -t phar conversion. The php plugin meta data fields are somewhat broader than composer bundle specs; and the dependencies specifically more localized application/api-targeted. Besides there's no agreed mapping yet between composer and system packages.)
Note that there's a distinct -s composer source package plugin. The two have orthogonal use cases.
-u lcase<xmp>=*</xmp>

Converts directory and filenames to lowercase.
- Directories are always lowercased.
- File extensions can be specified with -u lcase=* for all, or lcase=php:cpp for specific groups to lowercase.
This is meant just for the -t phar target. It's probably less suitable for system packages.
-u deps<xmp>=fedora</xmp>

State: incomplete

The deps filter is supposed to convert distro-specific package names. It utilizes whohas or distromatch. Since the latter is discontinued, it might make sense to just bundle a few package maps with xpm/fpm directly, and perform the lookup (for major distros) internally. (Would also be faster, really.)
-u unprefix

For extracing system package contents back into relative/local path structures you can use:
```
 xpm -s deb -u unprefix=/usr/share/appfiles/ -t zip pkgname.deb
```
This will slice out just the files given below the base path as filter= argument.
-u fixperms

State: redundant

Applies umask 0755 to files. Which is somewhat redundant since both deb and rpm plugins already implement a variation thereof.
-u strip

Strips debugging symbols from binaries.
- Is just applied to binaries.
- Skips shared libraries currently.
-u preprocess<xmp>=py</xmp>

Pipes source files through preprocess.py to expand macros. This is meant for scripting languages, not as C/C++ build hook.

The recognized comment syntax is based on the source language / file extensions.
- # #define
- // #ifdef
- /* #include "file.ph" */
- 
It doesn't process literal C-macros.
Notes
- In-code substitutions of defined constants are enabled per default (preprocess -s flag).
- Expects a file extension list, such as -u preprocess=pl:php:rb.
- Requires preprocess to be available, which is packaged for Debian distros at least.
-u packfile

To make packaging hooks even more flexible, there's now a thing called Packfiles. They're basically make scripts, get run within the /tmp/pack-xyz-staging/ directory, after all "source" files have been collected.

A main Packfile gets picked up from where fpm/xpm was invoked from. Any Packfiles that somehow end up in the staging path along with "source" files will be run too, but be removed afterwards. Both types run only within the staging dir, so can use relative paths all along:
```
 all:
        touch  etc/defaults/mypkg.conf
```
A few environment variables are predefined (PACK_TYPE, PACK_NAME, PACK_VERSION, PACK_ARCH) for even more cusomizations. But of course shouldn't be used habitually. The purpose of Packfiles is to untangle the real build steps from mere/minor packaging and file arrangement tasks.

update filters

`-u man`

`-u appdata`

`-u desktop`

`-u composer`

`-u lcase<xmp>=*</xmp>`

`-u deps<xmp>=fedora</xmp>`

`-u unprefix`

`-u fixperms`

`-u strip`

`-u preprocess<xmp>=py</xmp>`

`-u packfile`