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/doc/e1-data-model.txt b/openbsc/doc/e1-data-model.txt
index 9286d5f..509004f 100644
--- a/openbsc/doc/e1-data-model.txt
+++ b/openbsc/doc/e1-data-model.txt
@@ -140,7 +140,7 @@
 	union {
 		struct {
 			/* mISDN driver has one fd for each ts */
-			struct bsc_fd;
+			struct osmo_fd;
 		} misdn;
 	} driver;
 };