Write caches to first directory with permission. Valid cache in FcDirCacheOpen.

Write caches to first directory with permission. Valid cache in FcDirCacheOpen.

Previous policy was to attempt to update the cache in place and bail if that
didn't work. Now, search for the first writable directory and place the
cache file there instead. Furthermore, on startup, search directory list for
valid cache files instead of bailing if the first found cache file wasn't
valid.