Add EVENT_BASE_FLAG_EPOLL_DISALLOW_TIMERFD flag (fixes: libevent#958)

By default we are using CLOCK_MONOTONIC_COARSE, but if
EVENT_BASE_FLAG_PRECISE_TIMER isset, then CLOCK_MONOTONIC will be used,
however this will also enable timerfd, while this is not always what
someone wants, hence add a flag to control this (by default the old
behavior is preserved, set new flag to change it).

epoll.c

@@ -189,6 +189,7 @@ epoll_init(struct event_base *base)
 	  event_base, we can try to use timerfd to give them finer granularity.
 	*/
 	if ((base->flags & EVENT_BASE_FLAG_PRECISE_TIMER) &&
+	    !(base->flags & EVENT_BASE_FLAG_EPOLL_DISALLOW_TIMERFD) &&
 	    base->monotonic_timer.monotonic_clock == CLOCK_MONOTONIC) {
 		int fd;
 		fd = epollop->timerfd = timerfd_create(CLOCK_MONOTONIC, TFD_NONBLOCK|TFD_CLOEXEC);

include/event2/event.h

@@ -570,7 +570,20 @@ enum event_base_config_flag {
 	    however, we use less efficient more precise timer, assuming one is
 	    present.
 	 */
-	EVENT_BASE_FLAG_PRECISE_TIMER = 0x20
+	EVENT_BASE_FLAG_PRECISE_TIMER = 0x20,
+
+	/** With EVENT_BASE_FLAG_PRECISE_TIMER,
+	    epoll backend will use timerfd for more accurate timers, this will
+	    allows to disable this.
+
+	    That said that this is something in between lack of
+	    (CLOCK_MONOTONIC_COARSE) and enabled EVENT_BASE_FLAG_PRECISE_TIMER
+	    (CLOCK_MONOTONIC + timerfd).
+
+	    This flag has no effect if you wind up using a backend other than
+	    epoll and if you do not have EVENT_BASE_FLAG_PRECISE_TIMER enabled.
+	 */
+	EVENT_BASE_FLAG_EPOLL_DISALLOW_TIMERFD = 0x40,
 };
 
 /**