Handle EAGAIN like we do ENOMEM from poll() and select().

On some systems, poll() and select() can return EAGAIN instead
of ENOMEM if there is a kernel resource shortage.  In this case
we just re-enter the event loop and retry.

lib/util/event.c

@@ -661,7 +661,7 @@ rescan:
 	nready = sudo_ev_scan_impl(base, flags);
 	switch (nready) {
 	case -1:
-	    if (errno == ENOMEM)
+	    if (errno == ENOMEM || errno == EAGAIN)
 		continue;
 	    if (errno == EINTR) {
 		/* Interrupted by signal, check for sigevents. */

lib/util/event_poll.c

@@ -211,7 +211,7 @@ sudo_ev_scan_impl(struct sudo_event_base *base, int flags)
 	/* Error: EINTR (signal) or EINVAL (nfds > RLIMIT_NOFILE) */
 	sudo_debug_printf(SUDO_DEBUG_ERROR|SUDO_DEBUG_LINENO|SUDO_DEBUG_ERRNO,
 	    "sudo_ev_poll");
-	debug_return_int(-1);
+	break;
     case 0:
 	/* Front end will activate timeout events. */
 	sudo_debug_printf(SUDO_DEBUG_INFO, "%s: timeout", __func__);

lib/util/event_select.c

@@ -234,7 +234,7 @@ sudo_ev_scan_impl(struct sudo_event_base *base, int flags)
     switch (nready) {
     case -1:
 	/* Error or interrupted by signal. */
-	debug_return_int(-1);
+	break;
     case 0:
 	/* Front end will activate timeout events. */
 	break;