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.

compat.h

@@ -443,8 +443,8 @@ void		*recallocarray(void *, size_t, size_t, size_t);
 
 #ifdef HAVE_KEYCTL
 /* keyring-linux.c */
-void		 keyring_join(void);
-void		 keyring_revoke(void);
+void		 new_session_keyring(void);
+void		 revoke_session_keyring(void);
 #endif
 
 #ifdef HAVE_SYSTEMD