Replace the session-keyring option with a configure option

Join the new session keyring when the server starts rather than once the
configuration file has been loaded, and control it at build time with
--enable-keyring, on by default if linux/keyctl.h is found.

Rename keyring_join and keyring_revoke to new_session_keyring and
revoke_session_keyring.

server.c

@@ -220,6 +220,10 @@ server_start(struct tmuxproc *client, uint64_t flags, struct event_base *base,
 	TAILQ_INIT(&message_log);
 	gettimeofday(&start_time, NULL);
 
+#ifdef HAVE_KEYCTL
+	new_session_keyring();
+#endif
+
 #ifdef HAVE_SYSTEMD
 	server_fd = systemd_create_socket(flags, &cause);
 #else
@@ -260,7 +264,7 @@ server_start(struct tmuxproc *client, uint64_t flags, struct event_base *base,
 	job_kill_all();
 	prompt_save_history();
 #ifdef HAVE_KEYCTL
-	keyring_revoke();
+	revoke_session_keyring();
 #endif
 
 	exit(0);