MINOR: fd: Add fd_active function

This inlined function is used to check if a fd is active for receive or send. It
will ease threads support integration.
diff --git a/include/proto/fd.h b/include/proto/fd.h
index 38c12b6..526f895 100644
--- a/include/proto/fd.h
+++ b/include/proto/fd.h
@@ -241,6 +241,14 @@
 	return (unsigned)fdtab[fd].state & FD_EV_POLLED_W;
 }
 
+/*
+ * returns true if the FD is active for recv or send
+ */
+static inline int fd_active(const int fd)
+{
+	return (unsigned)fdtab[fd].state & FD_EV_ACTIVE_RW;
+}
+
 /* Disable processing recv events on fd <fd> */
 static inline void fd_stop_recv(int fd)
 {