Add command to de-keyword package and reverse dependencies

[matoro]

Per discussion with @mattst88 , there should be a command to allow a user to quickly de-keyword a package along with all its reverse dependencies. In the scenario where a package picks up a new unconditional dependency which is not keyworded on niche arches, frequently devs working on those niche arches will not get around to keywording the new dependency quickly enough. Often that leads to old versions being kept in tree, but this then results in inability to remove those old versions as they are the last ones providing keywords for said arches.

When a simple USE-mask is not sufficient, we should be encouraging the de-keywording of non-core packages on niche arches, where keywords can be later added back when a developer or arch-tester with interest has the time. Right now this is a manual process, leading to infrequent usage by maintainers, but an official nattka command would greatly ease this process and encourage it to be used more frequently.