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`.
Yeah I agree with @btm here. I think we can add a new method named similar to `set_members_list` does what @btm suggests and call that method from here.

That would make the code more readable and not require the complex `append` logic we need on the `manage` action.