MFC r270320:
Check the validity of struct sigaction sa_flags value, reject unknown
flags.
MFC r270321:
Ensure that sigaction flags for signal, which disposition is reset to
ignored or default, are not leaking.

MFC r270504:
Revert the handling of all siginfo sa_flags except SA_SIGINFO to the
pre-r270321 state.
MFC r270345:
In do_lock_pi(), do not override error from umtxq_sleep_pi() when
doing suspend check.