Move systemd_socket config to command line argument

[Jan Alexander Steffens]

Whether the daemon takes sockets from systemd or opens its own should be controlled via a command line argument (e.g. atuin daemon --systemd). Atuin's own config file is the wrong place for this.

This way starting the daemon using systemd can just work without having to manually edit Atuin's config first.

[Ellie Huxtable]

You could use the env setting, like this

ATUIN_DAEMON__SYSTEMD_SOCKET=true atuin daemon

All config fields have an env equivalent, for this sort of reason