Sigprocmask handling errors in nxsig_deliver()

[patacongo]

When delivering a signal, the signals specified in sa_mask are masked:

  /* Save a copy of the old sigprocmask and install the new
   * (temporary) sigprocmask.  The new sigprocmask is the union
   * of the current sigprocmask and the sa_mask for the signal being
   * delivered plus the signal being delivered.
   */

  savesigprocmask   = stcb->sigprocmask;
  sigorset(&newsigprocmask, &savesigprocmask, &sigq->mask);
  stcb->sigprocmask = newsigprocmask;

[modified by #9299]

However there are two problems when when the signal handler returns and the sigprocmask is restored as mentioned in the comments:

  /* Restore the original sigprocmask.
   *
   * What if the signal handler changed the sigprocmask?  Try to retain
   * any such changes here.
   *
   * REVISIT: This logic is imperfect.  It will fail to detect bits set
   * in the current sigprocmask that were already set by newsigprocmask.
   */

  nxsig_xorset(&tmpset1, &stcb->sigprocmask, &newsigprocmask);
  sigandset(&tmpset2, &stcb->sigprocmask, &tmpset1);
  nxsig_nandset(&tmpset1, &savesigprocmask, &tmpset1);
  sigorset(&stcb->sigprocmask, &tmpset1, &tmpset2);