UID/GID defined explicitly rather than from variables

[kirrus]

Currently, the script hardcodes uids and guids, specifically, see https://github.com/BigRedS/postfixadmin-installer/blob/master/postfixadmin-installer#L354

I'm not confident enough in my programming skills to fix this one, but I assume just adding usages of $uid and $gid would work?

[BigRedS]

I ... think this is fixed, but I also might not understand (or remember!) the problem. Is it?