(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).
I'll leave this as is until @joshcooper gave his opinion.

Please do be aware I have [this PR](https://github.com/apalmblad/ruby-shadow/pull/22) open which once it's been merged allows to add another method for the `loginclass`.