v3.1.0

This release adds support for bsdtar (compile-time option), enables
parallel building of delta packs with pack_maker.sh, adds an initial
functional test suite, and addresses a few bugs:

• Makes parallel pack generation more robust when building packs for
  multiple versions simultaneously.
• Fixes basic_creator.sh to work properly on first run.
• Removes an unneeded -a option passed to tar, and fixes tar argument
  ordering (files always after options).
• Makes hash calculation always account for xattrs, since swupd-client
  unconditionally accounts for them.