Add contrib/tor.service for use with systemd
Put tor.service in the right place, and autoconfify it

This closes 8368.
Quench gcc's complaints about discarding constness in TO_ORIGIN_CIRCUIT.

This was previously satisfied by using a temporary variable, but there
are three other instances in circuitlist.c that gcc is now bothered by,
so now introduce a CONST_TO_ORIGIN_CIRCUIT that takes a const
circuit_t instead.
changes file for last patch