systemd ships `units/serial-getty@.service.m4` with the `--keep-baud`
option.
We override that unit, and didn't add the `--keep-baud` option. (We have
it in our other getty options there).
Having `--keep-baud` in `serial-getty@` makes a lot of sense - the
console keeps working if it's initialized with a less standard baud
rate, such as the [Helios64](https://wiki.kobol.io/helios64/intro/).
This corresponds to agetty's --login-options argument.
With this change, I can set
services.getty.autologinUser = "qyliss";
services.getty.loginOptions = "-- \\u";
and have my username prefilled, but with my password still
required (unlike the normal autologinUser behaviour).
It's been 8.5 years since NixOS used mingetty, but the option was
never renamed (despite the file definining the module being renamed in
9f5051b76c1 ("Rename mingetty module to agetty")).
I've chosen to rename it to services.getty here, rather than
services.agetty, because getty is implemantation-neutral and also the
name of the unit that is generated.
