/*
* If the current process has received a signal (should be caught or cause
* termination, should interrupt current syscall), return the signal number.
*
* Stop signals with default action are processed immediately, then cleared;
* they aren't returned. This is checked after each entry to the system for
* a syscall or trap.
*
* We will also return -1 if the process is exiting and the current LWP must
* follow suit.
*/