pause_sbt(): Take the cold path (ie. use DELAY()) if KDB is active

This fixes a panic in the i915 driver when one uses debug.kdb.enter=1
under vt(4).

PR:		193269
Reported by:	emaste@
Submitted by:	avg@
MFC after:	3 days
- Make hhook_run_socket() vnet-aware instead of adding CURVNET_SET() around
  the function calls.
- Fix a memory leak and stats in the case that hhook_run_socket() fails
  in soalloc().

PR:	193265
Fix support for IPv6 nameservers.

PR:		188931
Submitted by:	Takefu <takefu@airport.fm>
MFC after:	3 days
Fail rather than segfault if neither PAM_TTY nor PAM_RHOST is set.

PR:		83099
MFC after:	3 days