Move get_grosysid() to utils/cache/lsyscache.c

Greetings,

  Small patch to move get_grosysid() from catalog/aclchk.c to
  utils/cache/lsyscache.c where it can be used by other things.  Also
  cleans up both get_usesysid() and get_grosysid() a bit.  This is in
  preparation for 'Group Ownership' support.

      Thanks,

        Stephen