Add support for requesting profile changes

[mattst88]

Ruby stabilizations almost never receive sanity-check+ because they require profiles changes to RUBY_TARGETS. See https://bugs.gentoo.org/688976 for example

If the package list field (which might need a rename :) could accept a syntax for profile changes we could do better checking of changes like this.

E.g., something like this, perhaps

use.stable.mask: -ruby_targets_ruby25
make.defaults: RUBY_TARGETS="ruby24 ruby25"

app-eselect/eselect-ruby-20190121
dev-ruby/did_you_mean-1.2.1
dev-ruby/rdoc-6.1.2
dev-lang/ruby-2.5.8

[mgorny]

I suppose it's start to thinking how to implement it. I suppose the first question is how fancy the support should be. Would it be sufficient to specify a file, and a change to be applied to it? Say, something like:

profiles/arch/hppa/use.mask: -gnome-keyring

that would append -gnome-keyring to the bottom? (which is kinda equivalent to removing gnome-keyring that is there)

[mattst88]

Yeah, I think that would work. I agree that there's a lot of ambiguity to just specifying use.mask: -gnome-keyring so specifying the whole path would be fine with me if that simplifies the problem.