bug#31611: Add a property in packages to refresh to a specific version

[Hartmut Goebel]

`guix refresh` always fetches the newest version of packages. This might
not be desired in all cases. E.g. one might want to keep some still
supported "major" upstream-version and not yet update to the next
"major" version.

Ludo suggested to add a property in packages that would instruct
updaters to restrict the version search space to a given regexp.

Rational: Example: KDE Plasma 5 has 5.12 as long-term-support (LTS)
version, while 5.14 has already being released. Do easy maintenance of
dependent packages, sticking to 5.12.x micht be desired.

Original discussion:
https://lists.gnu.org/archive/html/guix-devel/2018-02/msg00055.html

-- 
Regards
Hartmut Goebel

| Hartmut Goebel          | address@hidden               |
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