gplazma-nis: plugin should fail on invalid mapping

in set up like:

map      sufficient nis
map      sufficient  authzdb

nis plugin will always stop processing even if mapping is not found.

Ticket: #7946
Acked-by: Paul Millar
Target: trunk, 2.6, 2.7
Require-book: no
Require-notes: yes
Merge pull request #165 from kofemann/iss7946-2.7

Iss7946 2.7