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/tests/channel/channel_test.c b/openbsc/tests/channel/channel_test.c
index 258938c..e05af1e 100644
--- a/openbsc/tests/channel/channel_test.c
+++ b/openbsc/tests/channel/channel_test.c
@@ -68,7 +68,7 @@
 	subscr_get_channel(subscr, RSL_CHANNEED_TCH_F, subscr_cb, (void*)0x2342L);
 
 	while (1) {
-		bsc_select_main(0);
+		osmo_select_main(0);
 	}
 }