Move config to new upstream preferred name /etc/tlshd/config on upgrade

The standard config filename changed in 1.3.0 from /etc/tlshd.conf to /etc/tlshd/config. The old filename is still supported, but results in a warning on startup. It is also inconsistent with where the package will now install the config.

Use dpkg-maintscript-helper to move any existing config file in the correct way on upgrade.