libnfsidmap: id_as_chars() fails zero value ids.

Root has a zero value id which is valid and
should not be mapped to nfsnobody

Signed-off-by: Steve Dickson <steved@redhat.com>
nss: use strrchr() instead of strchr() to get the last occurrence of "@"

Signed-off-by: Shijoe Panjikkaran <spanjikk@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>