Implement put_channel/get_channel, save on which bts we currently operate

Allow to change the refcount for a given channel. Store which
bts is our primary bts. A command to switch the primary bts will
be added as well. This makes entering and parsing of commands more
easy.
diff --git a/include/openbsc/telnet_interface.h b/include/openbsc/telnet_interface.h
index 885cd64..228b623 100644
--- a/include/openbsc/telnet_interface.h
+++ b/include/openbsc/telnet_interface.h
@@ -32,6 +32,7 @@
 	struct llist_head entry;
 	struct gsm_network *network;
 	struct bsc_fd fd;
+	int bts;
 
 	int command;
 	char *imsi;