Fixed connect(2) errors processing on old Linuxes.

While connect(2) states that non-blocking connect should use EPOLLOUT:

  EINPROGRESS
    The socket is non-blocking and the connection cannot be completed
    immediately.  It is possible to select(2) or poll(2) for completion by
    selecting the socket for writing.  After select(2) indicates writability,
    use getsockopt(2) to read the SO_ERROR option at level SOL_SOCKET to
    determine whether connect() completed successfully (SO_ERROR is zero)
    or unsuccessfully (SO_ERROR is one of the usual error codes listed here,
    explaining the reason for the failure).

On connect error, Linux 2.6.32 (CentOS 6) may return EPOLLRDHUP, EPOLLERR,
EPOLLHUP, EPOLLIN, but not EPOLLOUT.

src/nxt_socket.c

@@ -300,6 +300,28 @@ nxt_socket_shutdown(nxt_task_t *task, nxt_socket_t s, nxt_uint_t how)
 }
 
 
+nxt_err_t
+nxt_socket_error(nxt_socket_t s)
+{
+    int        ret, err;
+    socklen_t  len;
+
+    err = 0;
+    len = sizeof(int);
+    /*  
+     * Linux and BSDs return 0 and store a pending error in the err argument; 
+     * Solaris returns -1 and sets the errno.
+     */  
+    ret = getsockopt(s, SOL_SOCKET, SO_ERROR, (void *) &err, &len);
+
+    if (nxt_slow_path(ret == -1)) {
+        err = nxt_errno;
+    }   
+
+    return err;
+}
+
+
 nxt_uint_t
 nxt_socket_error_level(nxt_err_t err)
 {