libnfsidmap: respect Nobody-User/Nobody-Group

Previous behavior of libnfsidmap was to do a name lookup of
nobody@DEFAULTDOMAIN (for both user and group), which does not match
the behavior of rpc.idmapd.

This patch makes libnfsidmap respect Nobody-User/Nobody-Group for
lookups, thus making the nfsidmap utility properly handle the case if
nobody@DEFAULTDOMAIN does not directly map to any user/group on the
system.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Signed-off-by: Steve Dickson <steved@redhat.com>