Slightly expand README

Block until targets are reached

And expand docs on how to make sure the locker is ready before the
target is reached.

Inhibit sleeping to ensure we have time to react

This is still imperfect; see linked article for additional upcoming
changes in v3.0.0.

go mod tidy

Add builds badge to README

Run builds on CI

Fix lock services not stopping after waking up

Run in session.slice

From systemd.special(7):

> session.slice
>   All essential services and applications required for the session should use
>   this slice. These are services that either cannot be restarted easily or where
>   latency issues may affect the interactivity of the system and applications.
>   This includes the display server, screen readers and other services such as
>   DBus or XDG portals. Such services should be configured to be part of this
>   slice by adding Slice=session.slice to their unit files.

Include a changelog

Improve the README

Fix bogus log message

Translate session unlock event to unlock.target

Improve the README a bit more

Tidy up makefile

Notify systemd when we've initialised successfully

Use go-systemd for service management

Make logging format systemd-friendly

Improve error handling and logging

Tidy up infinite loop

Configure pre-commit hooks