Fix creation of non-empty FreeBSD groups

When Chef creates a new FreeBSD group, the `pw groupadd` system command is
invoked. If the group is to have members at the time it is created,
prior to this change Chef would try to specify the new members with the `-m`
option. Unfortunately, `pw groupadd` has no `-m` option: one is supposed
to use `-M` instead. See the following excerpt from the `pw` man page:

```
pw [-V etcdir] groupadd [group|gid] [-C config] [-q] [-n group] [-g gid]
  [-M members] [-o] [-h fd | -H fd] [-N] [-P] [-Y]
pw [-V etcdir] groupdel [group|gid] [-n name] [-g gid] [-Y]
pw [-V etcdir] groupmod [group|gid] [-C config] [-q] [-n name] [-g gid]
  [-l name] [-M members] [-m newmembers] [-d oldmembers] [-h fd | -H fd]
  [-N] [-P] [-Y]
```

http://www.freebsd.org/cgi/man.cgi?query=pw&apropos=0&sektion=0&manpath=FreeBSD+9.3-RELEASE&arch=default&format=html

This commit lets Chef use `pw groupadd -M`.
These kind of arguments aren't ideal, as we have no context here as to what sending true or false means.

In the context of create_group, we just need the list of members, right? Could the first half of set_members_options be refactored into another method that returns this list, and then here we just add "-M #{that list}" to the list of command line options? Maybe we don't even need that logic and we just set something like `member_options = "-M #{@new_resource.members.join(',')}"` right here and be done. Then, add tests for that behavior ;)