Fix get/set users/groups in quota props via numeric id

Fix setting/getting users/groups in quota properties through
numeric identifier.  This support was accidentally disabled
in the original port by applying the HAVE_IDMAP wrapper macro
too broadly.

Fix obtained by moving #ifdef HAVE_IDMAP to exclude only
the part of code that really needs IDMAP.  Now zfs (get|set)
(user|group)quota@1000 works as expected.

Signed-off-by: Massimo Maggi <massimo@mmmm.it>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1147