Add support for systemd style socket activation.

Add support for systemd style socket activation.

If the LISTEN_FDS environment variable is set and tinc is run in the
foreground, tinc will use filedescriptors 3 to 3 + LISTEN_FDS for its listening
TCP sockets. For now, tinc will create matching listening UDP sockets itself.

There is no dependency on systemd or on libsystemd-daemon.