Add NetBSD groupadd/useradd

This adds netbsd support to the modules `useradd` and `groupadd`. The `user.info` salt module is currently missing on netbsd -- netbsd does support the `useradd` and `groupadd` commands, but not python's `spwd` module (similar to freebsd with #3321):

https://gist.github.com/agjohnson/5609511

The nbsd useradd command doesn't support the system (`-r`) flag. For now, this patch disregards the system flag, though I suppose forcing a low uid and `/sbin/nologin` shell would emulate a system user? If there is any input on this I can rework the patch to do something intelligent with the system flag.

Also, is it best to disable netbsd (freebsd, openbsd) on the `shadow` module with https://github.com/agjohnson/salt/commit/fd06ef830d580742cc5b4078db326ac697f43f09#L1R23?