select: 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/src/write_queue.c b/src/write_queue.c
index 0642aad..c357c3f 100644
--- a/src/write_queue.c
+++ b/src/write_queue.c
@@ -23,7 +23,7 @@
 
 #include <osmocom/core/write_queue.h>
 
-int write_queue_bfd_cb(struct bsc_fd *fd, unsigned int what)
+int write_queue_bfd_cb(struct osmo_fd *fd, unsigned int what)
 {
 	struct write_queue *queue;