capabilities: CAP_CHOWN might be required by many plugins opening UNIX sockets

But as the sockets will be created with the user/group of the running
process this might not be required as no change may be needed.

src/libcharon/plugins/smp/smp.c

@@ -739,6 +739,12 @@ plugin_t *smp_plugin_create()
 	private_smp_t *this;
 	mode_t old;
 
+	if (!lib->caps->keep(lib->caps, CAP_CHOWN))
+	{	/* required to chown(2) control socket */
+		DBG1(DBG_CFG, "smp plugin requires CAP_CHOWN capability");
+		return NULL;
+	}
+
 	INIT(this,
 		.public = {
 			.plugin = {