Squashed commit of the following:

    Replace all calls to sigprocmask() in the OS proper with calls to nxsig_procmask().

    sched/signal:  Add internal OS interface nxsig_procmask().  This internal interface is equivalent to the standard sigprocmask() used by applications except that it does not modify the errno value.  Also fixes a problem in that the original sigprocmask() was not setting the errno.