Be more intelligent about storing file user and group names

Be more intelligent about storing file user and group names
- User and group names contain an enormous amount of redundancy: typically
  most files and directories are owned by root:root. Store the each unique
  name just once into global cache and store indexes to the cache into
  rpmfi user + group arrays. This saves several megabytes of memory on
  a transaction of non-trivial size...
- The cache implementation is dumb, but it's not noticeably slower than
  uncached version, copying megs of strings around is not cheap either.
  Easy enough to replace with a hash-table or such if performace becomes
  and issue...