select: add functionality to check socket state

osmo_fd_register() is used to register socket file descriptors,
after registering a socket, there is no way to test if the socket
is still registered or actually registered at all.

This commit adds a new function osmo_fd_register_check() that can
be used to check in advance, if the socket fd is registered,
before performing further operations.

Change-Id: I48ec7098d6bba586c81bf0d5c9088108e2c081c6
diff --git a/include/osmocom/core/select.h b/include/osmocom/core/select.h
index 2753637..c2b51a4 100644
--- a/include/osmocom/core/select.h
+++ b/include/osmocom/core/select.h
@@ -1,6 +1,7 @@
 #pragma once
 
 #include <osmocom/core/linuxlist.h>
+#include <stdbool.h>
 
 /*! \defgroup select Select loop abstraction
  *  @{
@@ -35,6 +36,7 @@
 	unsigned int priv_nr;
 };
 
+bool osmo_fd_is_registered(struct osmo_fd *fd);
 int osmo_fd_register(struct osmo_fd *fd);
 void osmo_fd_unregister(struct osmo_fd *fd);
 int osmo_select_main(int polling);