capabilities: Only plugins that require CAP_NET_ADMIN demand it

The daemon as such does not require this capability.

src/libhydra/plugins/kernel_pfkey/kernel_pfkey_plugin.c

@@ -62,6 +62,12 @@ plugin_t *kernel_pfkey_plugin_create()
 {
 	private_kernel_pfkey_plugin_t *this;
 
+	if (!lib->caps->keep(lib->caps, CAP_NET_ADMIN))
+	{	/* required to open PF_KEY sockets */
+		DBG1(DBG_KNL, "kernel-pfkey plugin requires CAP_NET_ADMIN capability");
+		return NULL;
+	}
+
 	INIT(this,
 		.public = {
 			.plugin = {