xkbinfo: use (a conditional) libxkbregistry to parse the rules files for us

Fixes !79 (merged)

This time with build-time conditional support for libxkbregistry, otherwise near-identical to the patch from !79 (merged). A few minor style fixes and the refactoring as separate commits. The modified test shows that despite sort order changes, the output is the same before and after, with and without xkbregistry.