Improved epoll failures handling.

epoll changes are committed to the kernel before epoll_wait() or
on changes array overflow.  In the latter case if there are errors
epoll_wait() timeout was not set to zero.

This commit is related to #173 issue on GitHub.
Thanks to 洪志道 (Hong Zhi Dao).

src/nxt_event_engine.h

@@ -202,6 +202,8 @@ typedef struct {
     nxt_uint_t                    mchanges;
     int                           mevents;
 
+    uint8_t                       error;  /* 1 bit */
+
     nxt_epoll_change_t            *changes;
     struct epoll_event            *events;