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/gprs_ns.h b/openbsc/include/openbsc/gprs_ns.h
index f497e23..8f28d46 100644
--- a/openbsc/include/openbsc/gprs_ns.h
+++ b/openbsc/include/openbsc/gprs_ns.h
@@ -133,13 +133,13 @@
 
 	/* NS-over-IP specific bits */
 	struct {
-		struct bsc_fd fd;
+		struct osmo_fd fd;
 		uint32_t local_ip;
 		uint16_t local_port;
 	} nsip;
 	/* NS-over-FR-over-GRE-over-IP specific bits */
 	struct {
-		struct bsc_fd fd;
+		struct osmo_fd fd;
 		uint32_t local_ip;
 		int enabled:1;
 	} frgre;