fix User/GroupField superclass
change services.models to use the new userfield
write custom form/model field for users/group to avoid code duplication
set default user/group to nobody for rsyncmod
fix default value for migration; fix typo
a few tweaks for rsyncmod + choices
rsyncmod modelform
middleware to restart and check whether rsyncd is running
create a rsyncd object on migration as well
add rsync service to services table in migration