nfs-utils: add and use nfs_authsys_create

The current mount, umount and showmount code uses
authunix_create_default to get an auth handle. The one provided by glibc
returned a truncated list of groups when there were more than 16 groups.
libtirpc however currently does an abort() in this case, which causes
the program to crash and dump core.

nfs-utils just uses these auth handles for the MNT protocol, so the
group list doesn't make a lot of difference here. Add a new function
that creates an auth handle with a supplemental gids list that consists
only of the primary gid. Have nfs-utils use that function anywhere that
it currently uses authunix_create_default. Also, have the caller
properly check for a NULL return from that function.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>