Allow '@' signs in user names

[squell]

Closing #1064

[3v1n0]

I think we should be a bit more conform to what adduser (see man adduser.conf) allows here:

VALID NAMES
       Historically, adduser(8) and addgroup(8) enforced conformity to IEEE Std 1003.1-2001, which allows only the following characters to ap‐
       pear in group- and usernames: letters, digits, underscores, periods, at signs (@) and dashes.  The name may not start with a dash or @.
       The "$" sign is allowed at the end of usernames to allow typical Samba machine accounts.

       The  default settings for NAME_REGEX and SYS_NAME_REGEX allow usernames to contain lowercase letters and numbers, plus dash (-) and un‐
       derscore (_); the name must begin with a letter (or an underscore for system users).

       The least restrictive policy, available by using the --allow-all-names option, simply makes the same checks as useradd(8): cannot start
       with a dash, plus sign, or tilde; and cannot contain a colon, comma, slash, or whitespace.

       This option can be used to create confusing or misleading names; use it with caution.

       Please note that regardless of the regular expressions used to evaluate the username, it may be a maximum of 32 bytes; this may be less
       than 32 visual characters when using Unicode glyphs in the username.

[squell]

Todo: add unit tests, add comment about $ being last.