table virtual_users:
id int
parent_id int (contains a valid `id` value)
username varchar
pw_crypt varchar
pw_htpasswd varchar
pw_clear varchar


table unix_users:
unix int (contains a valid unix uid)
virtual int (contains a valid virtual_users.id value)


table service_settings:
user int (contains a valid unix uid)
service varchar (e.g. "ftp")
permission varchar (e.g. "username", "read_only")
value varchar (value for the permission setting)