Expand and expose ctrl connection allocation

Add function for allocating CTRL connection to public headers and
replace call to previous static function with it. Add doxygen docs for
this function.

It's useful if we need to allocate ctrl connection but don't need to
bind to any interfaces: when we act as ctrl client.

Related: OS#1615
Change-Id: I522ed809cbebfd3d7dd08b4ed9137b39ff192e32
diff --git a/include/osmocom/ctrl/control_if.h b/include/osmocom/ctrl/control_if.h
index 512ae10..f2af1db 100644
--- a/include/osmocom/ctrl/control_if.h
+++ b/include/osmocom/ctrl/control_if.h
@@ -27,5 +27,5 @@
 					       const char *bind_addr,
 					       uint16_t port,
 					       ctrl_cmd_lookup lookup);
-
+struct ctrl_connection *osmo_ctrl_conn_alloc(void *ctx, void *data);
 int ctrl_cmd_handle(struct ctrl_handle *ctrl, struct ctrl_cmd *cmd, void *data);