Fix assertion failure in nslookup/dig/mdig when message has multiple SIG(0) options.

Fix assertion failure in nslookup/dig/mdig when message has multiple SIG(0) options.

When parsing message with DNS_MESSAGE_BESTEFFORT (used exclusively in
tools, never in named itself) if we hit an invalid SIG(0) in wrong
place we continue parsing the message, and put the sig0 in msg->sig0.
If we then hit another sig0 in a proper place we see that msg->sig0
is already 'taken' and we don't free name and rdataset, and we don't
set seen_problem. This causes an assertion failure.
This fixes that issue by setting seen_problem if we hit second sig0,
tsig or opt, which causes name and rdataset to be always freed.

(cherry picked from commit 51a55ddbb73f8707de3d1b8cda15c8f61585bacb)