user creates low-numbered uids on OS X (ignores 'system')

Looking at the source code, the default for `uid` is `next_system_id` regardless of the `system` attribute. `next_system_id` does what it promises, in fact quite well – it will select the lowest possible id over 20.

It's quite possible that such low id's will conflict with other user accounts from Apple. It also seems that system users cannot be associated with file shares.