Give the server its own session keyring on Linux

The server otherwise keeps the session keyring of the login session that
started it, which pam_keyinit(8) may revoke when that session ends,
leaving the server and its panes unable to use the kernel keyring. Join
a new session keyring with the user keyring linked in once the
configuration file has been loaded, and revoke it when the server exits.

This is controlled by a new session-keyring server option, on by
default.

server.c

@@ -259,6 +259,9 @@ server_start(struct tmuxproc *client, uint64_t flags, struct event_base *base,
 
 	job_kill_all();
 	prompt_save_history();
+#ifdef HAVE_KEYCTL
+	keyring_revoke();
+#endif
 
 	exit(0);
 }