Revert the handling of all siginfo sa_flags except SA_SIGINFO to the
pre-r270321.  Namely, the flags are preserved for SIG_DFL and SIG_IGN
dispositions.

Requested and reviewed by:	jilles
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270504 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
Optimize f_which() to be slightly faster still.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@270505 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f