Remove all "unreachable" objects in store

[Hartmut Goebel <h.goebel@crazy-compilers.com>]

Hi,

when packaging software, the store fills up with intermediate versions.
I would like to get rid of these.

My "production" environment typically lags behind the development head
quite some releases. So if I run `guix gc`, this would remove quite a
lot of packages which will then be rebuild just afterwards, when I build
the next package. Some of these are quite big, e.g. numpy or glibc.

So I'd like to garbage collect all packages which are no longer current,
for which there is no longer a way to build them.

Any ideas?

-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | h.goebel@crazy-compilers.com               |
| www.crazy-compilers.com | compilers which you thought are impossible |