MFC r283471:

MFC r283471:

According to Linux man sigaltstack(3) shall return EINVAL if the ss
argument is not a null pointer, and the ss_flags member pointed to by ss
contains flags other than SS_DISABLE. However, in fact, Linux also
allows SS_ONSTACK flag which is simply ignored.

For buggy apps (at least mono) ignore other than SS_DISABLE
flags as a Linux do.

While here move MI part of sigaltstack code to the appropriate place.

git-svn-id: svn://svn.freebsd.org/base/stable/10@293572 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f