For sigaction(2), ignore possible garbage in sa_flags for sa_handler

SIG_DFL or SIG_IGN. Sloppy code does not fully initialize struct

sigaction for such cases, and being too demanding in the case of
default handler does not catch anything.

Reported and tested by: Alex Tutubalin <lexa@lexa.ru>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week