ruby_select experimental implementation

[Rainer Müller]

C. Florian Ebeling wrote:
> One other thing I noticed is that ruby_select does not
> declare any dependency on any ruby, which surprised me
> a bit. But python does not require any python either.
> There is probably some reasoning behind this. I would
> like to learn more about the this.

I would find it a bit strange if *_select required a specific version.
Instead, the ruby and python versions should require *_select.

See <http://trac.macports.org/ticket/19126>

> These two behaviours together, though, leave users in the
> situation where they install ruby_select, then issue
> 
>   sudo ruby_select ruby186
> 
> which does not complain, but there won't be a usable ruby
> afterwards, only a couple of symlinks pointing into the nowhere.

As I said before, the select files should be part of the corresponding
ports and not of *_select. This way it is only possible to select
versions which are installed and active.

See the pythonXX ports as an example using the select port group.

Rainer