Use upstream version of fstrim instead of the local one

Use upstream version of fstrim instead of the local one

Local version of fstrim was dropped so that we depend on upstream
version which is now available via FSTRIM_PROG. _require_fstrim was
added to check if fstrim is available in the system and
_test_batched_discard to check if we can run fstrim on certain
mountpoint.

Also tests 251 and 260 were modified to reflect this change.

Signed-off-by: Tomas Racek <tracek@redhat.com>
Reviewed-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>