poll-loop: Enable checking whether a FD caused a wakeup.

Each time we run through the poll loop, we check all file descriptors
that we were waiting on to see if there is data available.  However,
this requires a system call and poll already provides information on
which FDs caused the wakeup so it is inefficient as the number of
active FDs grows.  This provides a way to check whether a given FD
has data.

lib/netlink-socket.c

@@ -668,6 +668,14 @@ nl_sock_wait(const struct nl_sock *sock, short int events)
     poll_fd_wait(sock->fd, events);
 }
 
+/* Checks whether this socket caused a wakeup in the previous run of the poll
+ * loop. */
+short int
+nl_sock_woke(const struct nl_sock *sock)
+{
+    return poll_fd_woke(sock->fd);
+}
+
 /* Returns the PID associated with this socket. */
 uint32_t
 nl_sock_pid(const struct nl_sock *sock)