K 10
svn:author
V 3
kib
K 8
svn:date
V 27
2016-07-12T03:53:15.926076Z
K 7
svn:log
V 696
Revive the check, disabled in r197963.

Despite the implication (process has pending signals -> the current
thread marked for AST and has TDF_NEEDSIGCHK set) is not true due to
other thread might manipulate its signal blocking mask, it should still
hold for the single-threaded processes.  Enable check for the condition
for single-threaded case, and replicate it from userret() to ast() as
well, where we check that ast indeed has no signal to deliver.

Note that the check is under DIAGNOSTIC, it is not enabled for INVARIANTS
but !DIAGNOSTIC since it imposes too heavy-weight locking for day-to-day
used debugging kernel.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week

END