Use "lima" username if the local user is not a valid Linux name

[jandubois]

At least some companies set local users via directory services and choose the email address as the user name. The '@' character is not valid in Linux usernames.

Fixes #191

[jandubois]

For testing I've forced the username to be an email address like this:

+                       cache.u.Username = "user@example.com"
+                       if err := identifiers.Validate(cache.u.Username); err != nil {

This is the output when starting an instance:

$ l start default
INFO[0000] Using the existing instance "default"
WARN[0000] local user "user@example.com" is not a valid Linux user name: identifier "user@example.com" must match ^[A-Za-z0-9]+(?:[._-](?:[A-Za-z0-9]+))*$: invalid argument; using "lima" username instead
[...]
INFO[0023] READY. Run `lima` to open the shell.

$ lima id
uid=501(lima) gid=1000(lima) groups=1000(lima)

[AkihiroSuda]

Thanks