Support for defining multiple FTP-Users per domain

This patch allows a domain administrator to define a new configuration
file for each domain, "config/ftp-users" which allows them to specify an
arbitrary number of additional FTP users for the domain.

This config-file is in the format:

username:password:dir:mode:quota

* This file consists of up to 5 colon-separated fields per line. dir,
   mode and quota are optional
* The username should be a sensible ASCII string
* The password can either be a plaintext string or an encrypted password
  from /usr/bin/symbosis-encrypt-password
* The dir should be a path relative to the domain's "public" dir (e.g
  htdocs/userfiles would give the user access *only* to
  /srv/domain.tld/public/htdocs/userfiles)
* Mode is currently unused, but can be an empty string ("::")
* Quota can be a sensible quota as a string (e.g "100M", "32G")