add username prompt, reservedUsernames config, minor cleanup

[nmschulte]

I created this work for Mobian, and as suggested am sharing here for review and inclusion. A question is open about where the reserved usernames list comes from; see https://salsa.debian.org/Mobian-team/packages/calamares-extensions/-/merge_requests/4.

... Sharing my WIP for early review; should the username prompt be a separate screen, or is it acceptable to add it to the password prompt screen?
I also wonder how to properly attribute my work ...

[adriaandegroot]

I don't have a particular comment to make here, since the module is mobile-(mobian)specific and I don't feel qualified to comment. Maybe @ollieparanoid or @PureTryOut ? Those are Postmarket folks, I think.

I don't know what the source is of the reserved names. It seems to contain a bunch that I wouldn't call reserved in the context of a general OS (cyrus, for instance, unless cyrus imap is always on the system). Moving the reserved list to C++ makes it much harder to adapt to whatever mobile OS is using this module, though.

[PureTryOut]

It seems fine, although I agree on the reserved usernames comment.
The module is mobile-specific, but not Mobian-specific 😉

[nmschulte]

I'll fix the TODO about the validation. Any thoughts about the placeholder TODO?
The reserved usernames change allows them to be configurable -- I can neuter the default list in the C++ code which IIUC is required though I'm not familiar with the system but came up with this from the other patterns; recommendations?

[ollieparanoid]

LGTM