If check_deferred_signal() execution needs binding of PLT symbol,
unlocking the rtld bind lock results in the processing of ast and
recursing into the check_deferred_signal().  Nested execution of
check_deferred_signal() delivers the signal to user code and clears
si_signo.  On return, top-level check_deferred_signal() frame
continues delivering the same signal one more time, but now with zero
si_signo.

Fix this by adding a flag to indicate that deferred delivery is
running, so check_deferred_signal() should avoid doing anything. Since
user signal handler is allowed to modify the passed machine context to
make return from the signal handler to cause arbitrary jump, or do
longjmp(). For this case, also clear the flag in thr_sighandler(),
since kernel signal delivery means that nested delivery code should
not run right now.

Reported by:	Vitaly Magerya <vmagerya@gmail.com>
Reviewed by:	davidxu, jilles
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week