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`.
OK, I inlined the `member_options = ...` into `create_group` as @btm suggested.

I played with getting rid of the `append` logic but the tests started failing and I realized I do not understand what is going on there, so I reverted it to the state it was on master.