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/src/osmo-bsc_nat/bsc_nat_utils.c b/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c
index 9ddc948..e796b2b 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c
@@ -787,7 +787,7 @@
 	return con_to_ctr[conn->con_type];
 }
 
-int bsc_write_cb(struct bsc_fd *bfd, struct msgb *msg)
+int bsc_write_cb(struct osmo_fd *bfd, struct msgb *msg)
 {
 	int rc;