abuild: add code for option extract-dir

It makes the unpack function extract to a file name dependent directory. It is useful to enable maintainer to work with archives that contain all files and directories in the root directory of the archives directly, instead of a subdirectory.

That'd enable me to remove all the custom unpack functions from the kopano-* packages. Other packages use them for the purpose of extracting to specific dirs, too.

The following packages do that:

• testing/font-fantasque-sans
• non-free/chromium-widevine
• main/docbook-xml

From a first glance, adding support for that directly would enable removal of the custom unpack function from those packages.