(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'm guessing that this came from the useradd provider, but I would rather have this unrolled than metaprogrammed. We can pull out lines 40 through 46 into a method that accepts either `:expiry` or `:password`, and have `expiry` and `shadow` methods that just invoke the common method with the appropriate value as the argument.

This is just my opinion though, would be good to have another opinion on this.