Make SIGTSTP work as expected if received during iopen.

In ca59eda iopen was created
which handles SIGINT received during a blocked open(). But as
noted in the commit message, SIGTSTP was handled the same as SIGINT.
This commit fixes SIGTSTP to stop the process rather than kill it.
This required changing psignals() to send SIGSTOP rather than SIGTSTP
to the current process to stop it. SIGTSTP doesn't stop the process
even though the handler is set to SIG_DFL. I don't know why, but
it seems harmless enough to use SIGSTOP rather than SIGTSTP.

os.c

@@ -323,9 +323,14 @@ public ssize_t iread(int fd, unsigned char *buf, size_t len)
 public int iopen(constant char *filename, int flags)
 {
 	int r;
-	if (!opening && SET_JUMP(open_label))
+	while (!opening && SET_JUMP(open_label))
 	{
 		opening = FALSE;
+		if (sigs & S_STOP)
+		{
+			psignals(); /* Stop the process */
+			continue;
+		}
 		sigs = 0;
 #if HAVE_SETTABLE_ERRNO
 #ifdef EINTR

signal.c

@@ -254,7 +254,16 @@ public void psignals(void)
 		LSIGNAL(SIGTTOU, SIG_DFL);
 #endif
 		LSIGNAL(SIGTSTP, SIG_DFL);
+		/*
+		 * {{ For reasons I don't understand, sending SIGTSTP does not stop
+		 *    the process if called from iopen, even though we set SIGTSTP
+		 *    to SIG_DFL above. So we use SIGSTOP if possible. }}
+		 */
+#ifdef SIGSTOP
+		kill(getpid(), SIGSTOP);
+#else
 		kill(getpid(), SIGTSTP);
+#endif
 		/*
 		 * ... Bye bye. ...
 		 * Hopefully we'll be back later and resume here...