(PUP-3605) use correct usermod flag on OpenBSD for group management

this allows 'membership => inclusive' to remove groups if needed as
'-G' only appends on OpenBSD and doesn't set.

The spec test is slightly unrelated in that it ensures the creation still uses `-G`, but currently there are no tests available for `objectadd.rb` (should be added, but it's out of scope for this PR).
This idea is exactly what I've been thinking off. When working on something unrelated in the `useradd` provider I found myself switching on `:osfamily` once more.

Aside from that, the '-G' flag is used on OpenBSD, but only for `useradd`, not `usermod`. So chose to add this code close to the `modifycmd`.

Perhaps the time has come to write new user/group provider for OpenBSD that also overrides `modifycmd`.