pkgsubmit.php: Parse .AURINFO metadata

pkgsubmit.php: Parse .AURINFO metadata

This allows for adding a metadata file called ".AURINFO" to source
tarballs to overwrite specific PKGBUILD fields. .AURINFO files are
parsed line by line. The syntax for each line is "key = value", where
key is any of the following field names:

* pkgname
* pkgver
* pkgdesc
* url
* license
* depend

Multiple "depend" lines can be specified to add multiple dependencies.

This format closely matches the .PKGINFO format that is used for binary
packages in pacman/libalpm. It can be extended by field name prefixes or
sections to support split packages later.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>