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.

compat.h

@@ -441,6 +441,12 @@ void		*reallocarray(void *, size_t, size_t);
 void		*recallocarray(void *, size_t, size_t, size_t);
 #endif
 
+#ifdef HAVE_KEYCTL
+/* keyring-linux.c */
+void		 keyring_join(void);
+void		 keyring_revoke(void);
+#endif
+
 #ifdef HAVE_SYSTEMD
 /* systemd.c */
 int		 systemd_activated(void);