src: use namespace prefix osmo_fd* and osmo_select*

Summary of changes:

s/struct bsc_fd/struct osmo_fd/g
s/bsc_register_fd/osmo_fd_register/g
s/bsc_unregister_fd/osmo_fd_unregister/g
s/bsc_select_main/osmo_select_main/g
diff --git a/openbsc/include/openbsc/osmo_bsc_rf.h b/openbsc/include/openbsc/osmo_bsc_rf.h
index f1ff9ba..dbbb3db 100644
--- a/openbsc/include/openbsc/osmo_bsc_rf.h
+++ b/openbsc/include/openbsc/osmo_bsc_rf.h
@@ -9,7 +9,7 @@
 struct osmo_bsc_rf {
 	/* the value of signal.h */
 	int policy;
-	struct bsc_fd listen;
+	struct osmo_fd listen;
 	struct gsm_network *gsm_network;
 
 	const char *last_state_command;