[Bug 2122047] [NEW] Case-fold the Windows username before sanitization

[Launchpad Bug Tracker]

You have been subscribed to a public bug by Carlos Nihelton (cnihelton):

The `wsl-setup` script reads the Windows user name and sanitizes it,
dropping invalid characters for a Linux user name, including ASCII upper
case letters. We could case fold the username first and then drop the
invalid ones, making it a bit less surprising. That would still have
some nasty cases we cannot handle in a smart way, like `MartínJoão` being
converted into `martnjoo` but that’s why we prompt for the username, to
let users edit that before actual submission.

While case folding can be done with  `tr "[:upper:]" "[:lower:]"` or even
better `awk '{print tolower(0)}'` (awk handles Unicode better), better yet
is to leverage PowerShell because then we're guaranteed to respect the
Windows user locale (a WSL instance could be set up with a different
locale and the case-folding results couldn't match the user's
expectation).

To illustrate how this works:

```bash
j at DESKTOP-551PQ9O:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 20.04.6 LTS
Release:        20.04
Codename:       focal

j at DESKTOP-551PQ9O:~$ powershell.exe -Command '$Env:Username'
João

j at DESKTOP-551PQ9O:~$ var="Username"; powershell.exe -NoProfile -Command '& {
[Console]::OutputEncoding = [System.Text.Encoding]::UTF8
$Env:'"${var}"'}'
João

j at DESKTOP-551PQ9O:~$ var="Username.ToLower()"; powershell.exe -NoProfile -Command '& {
[Console]::OutputEncoding = [System.Text.Encoding]::UTF8
$Env:'"${var}"'}'
joão

```

In the example above the username would be sanitized to: `oo`
but with the changes it would at least be: `joo`
(the 'ã' is still dropped, but it's much easier to add it).

The motivation for this change came from a user named `AdminUser`.
That would originally be sanitized to: `dminser`
but now it becomes: `adminuser`

One must remember that in the end this name is just a suggestion
prefilling a read input. User is free to edit it before hitting enter
and submitting the username to the `adduser` program.

** Affects: wsl-setup (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: wsl-setup (Ubuntu Resolute)
     Importance: Undecided
         Status: New

-- 
Case-fold the Windows username before sanitization
https://bugs.launchpad.net/bugs/2122047
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