Since following a delegation resets most fetch context state, address
marks (FCTX_ADDRINFO_MARK) set inside lib/dns/resolver.c are not
preserved when a delegation is followed.  This is fine for full
recursive resolution but when named is configured with "forward first;"
and one of the specified forwarders times out, triggering a fallback to
full recursive resolution, that forwarder should no longer be consulted
at each delegation point subsequently reached within a given fetch
context.

Add a new badnstype_t enum value, badns_forwarder, and use it to mark a
forwarder as bad when it times out in a "forward first;" configuration.
Since the bad server list is not cleaned when a fetch context follows a
delegation, this prevents a forwarder from being queried again after
falling back to full recursive resolution.  Yet, as each fetch context
maintains its own list of bad servers, this change does not cause a
forwarder timeout to prevent that forwarder from being used by other
fetch contexts.