swanctl: Read default socket from swanctl.socket option

Also read from swanctl.plugins.vici.socket so we get
libstrongswan.plugins.vici.socket if it is defined.

Fixes #2372.

conf/options/swanctl.opt

@@ -1,2 +1,5 @@
 swanctl.load =
-	Plugins to load in swanctl.
+	Plugins to load in swanctl.
+
+swanctl.socket = unix://${piddir}/charon.vici
+	VICI socket to connect to by default.

src/swanctl/command.c

@@ -315,6 +315,10 @@ int command_dispatch(int c, char *v[])
 {
 	int op, i;
 
+	uri = lib->settings->get_str(lib->settings, "%s.socket",
+			lib->settings->get_str(lib->settings, "%s.plugins.vici.socket",
+								   NULL, lib->ns), lib->ns);
+
 	options = options_create();
 	atexit(cleanup);
 	active = help_idx = registered;