Added system tests, writing which only became possible after adding a -i
command line switch forcing interactive mode.  nsupdate already bails on
error in non-interactive mode, so no further command line switches were
added.

rt28194 was rebased as agreed with Mark on Jabber back in June, due to
conflicting changes to bin/nsupdate/nsupdate.c and the nsupdate system
test which were merged in the meantime.

Please review.