romio: clean up file system detection

ROMIO file system checking had grown out of hand:
- use AC_CHECK_MEMBERS instead of our ancient home-grown stat member
checking logic.
- abstract the "stat/statfs/statvfs" part from the rest of the code so
  we don't duplicate detection logic in three places
- add a 'magic number' to the 'fstypes' struct so we can use that
  table multiple ways.

check for more file systems