kernel - Fix signal masking race assertion panic w/vkernel

kernel - Fix signal masking race assertion panic w/vkernel

* sigsuspend() and pselect() record the old signal mask in order to
  allow an interrupting signal to run its handler before the old mask
  is restored.

* When multiple threads are present a race can ensue where another thread
  changes the signal handler after sigsuspend() or pselect() have interrupted,
  but before they are able to process the signal.

* If the signal is no longer enabled the old signal mask is not restored
  on system call return, resulting in an assertion and panic.

* Fix the problem by checking the flag and restoring the old signal mask
  on return (rather than asserting when the flag is found to be non-zero
  on return).

Reported-by: Venkatesh Srinivas