Use poll() for connect loop

So we can exit nicely when asked to.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/server.h b/server.h
index 879faab..aed61b2 100644
--- a/server.h
+++ b/server.h
@@ -2,5 +2,6 @@
 #define FIO_SERVER_H
 
 extern int fio_server(void);
+extern int exit_backend;
 
 #endif