feat: support and auto-detect custom /c mountpoints

[jonaskuske]

fix #9

• if an /etc/wsl.conf config file exists, the install script now tries to check and use its root value.
  Comments beginning with # are ignored and indentation/whitespace should work fine I think

• $MNT (whether it's the default /mnt/c, ${WSL_CONF_ROOT}c or some custom mount setup specified during the install process) is now stored as win_mnt in /etc/pam_wsl_hello/config

• the Rust code uses that value as current directory when spawning the Authenticator instead of always using /mnt/c and throwing on other setups

Feel free to adjust or critique as you wish – I never have used Rust before so I was a bit stumbling in the dark throughout this... But it compiles and works 😅

Tested on Ubuntu 20.04 / WSL2

[nullpo-head]

Hi, thanks so much for your contribution! I haven't be able to take my time to test your change and release the new version yet, but will do that soon.