Validate cache contents and skip broken caches, looking down cache path for
valid ones.

Every time a directory is scanned, it will be written to a cache file if
possible, so fc-cache doesn't need to re-write the cache file. This makes
detecting when the cache was generated a bit tricky, so we guess that if the
cache wasn't valid before running and is valid afterwards, the cache file
was written.

Also, allow empty charsets to be serialized with null leaves/numbers.

Eliminate a leak in FcEdit by switching to FcObject sooner.

Call FcFini from fc-match to make valgrind happy.