groupadd: fix and implement missing functions for OpenBSD

gpasswd is only available on Linux and groupadd(8) does not support `-r'
on OpenBSD.
Add OpenBSD support for deluser() and memebers().
Merge branch 'obsd-groupadd' of https://github.com/ajacoutot/salt into ajacoutot-obsd-groupadd
fix lint errors
Merge pull request #15328 from thatch45/ajacoutot-obsd-groupadd

Ajacoutot obsd groupadd