Do NOT exit event loop on timer/signal watcher error

This patch fixes a blocking issue discovered in the Finit project.
At shutdown, or when there is any problem with a timer or signal
watcher, the event loop is abruptly terminated.

Instead of unconditionally exiting the event loop on problems with a
single watcher, invalidate the watcher and run the callback one last
time with UEV_ERROR in the events argument.

This is a semantic change wrt. watchers and their callbacks, so the
major version number will be bumped accordingly to signal the severity
of this change.  From now on it is STRONGLY recommended that each call
back function check for UEV_ERROR.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

src/private.h

@@ -38,7 +38,7 @@ typedef enum {
 } uev_type_t;
 
 /* Event mask, used internally only. */
-#define UEV_EVENT_MASK  (UEV_READ | UEV_WRITE | UEV_PRI | UEV_HUP | UEV_RDHUP | UEV_EDGE | UEV_ONESHOT)
+#define UEV_EVENT_MASK  (UEV_ERROR | UEV_READ | UEV_WRITE | UEV_PRI | UEV_HUP | UEV_RDHUP | UEV_EDGE | UEV_ONESHOT)
 
 /* Main libuEv context type */
 typedef struct {