watcher: Change handling of POLLERR and remove WATCHER_EXCEPT

We can't actually explicitly listen for errors by passing POLLERR in
`events` (the man page for poll() clearly states it's ignored).  On the
other hand, POLLERR can be returned for any FD and, even worse, it might
be the only event indicated.

The latter caused an infinite loop as we didn't notify the callback nor
clear the error by calling `getsockopt(..., SOL_SOCKET, SO_ERROR, ...)`.
And while the latter would be able to reset the state to break the loop,
it seems to leave the FD in a defunct state where no further events will
be returned by poll().  Notifying the callback works better (the error
is then reported by e.g. recvfrom()) and automatically happened already
if POLLERR was returned together with e.g. POLLIN.

So we now treat POLLERR like the other error indicators we handle (POLLHUP
and POLLINVAL) and just notify the callbacks.

src/libstrongswan/processing/watcher.h

@@ -55,7 +55,6 @@ typedef bool (*watcher_cb_t)(void *data, int fd, watcher_event_t event);
 enum watcher_event_t {
 	WATCHER_READ = (1<<0),
 	WATCHER_WRITE = (1<<1),
-	WATCHER_EXCEPT = (1<<2),
 };
 
 /**