MINOR: pollers: Add a way to wake a thread sleeping in the poller.

Add a new pipe, one per thread, so that we can write on it to wake a thread
sleeping in a poller, and use it to wake threads supposed to take care of a
task, if they are all sleeping.
diff --git a/src/cli.c b/src/cli.c
index d028429..739107d 100644
--- a/src/cli.c
+++ b/src/cli.c
@@ -890,6 +890,7 @@
 			     (fdt.iocb == dgram_fd_handler) ? "dgram_fd_handler" :
 			     (fdt.iocb == listener_accept)  ? "listener_accept" :
 			     (fdt.iocb == thread_sync_io_handler) ? "thread_sync_io_handler" :
+			     (fdt.iocb == poller_pipe_io_handler) ? "poller_pipe_io_handler" :
 			     "unknown");
 
 		if (fdt.iocb == conn_fd_handler) {