RFE: set up signing (semi-)automatically

[pmatilai]

We want all packages coming out of rpmbuild to be signed in 6.0 (#1256, #1573) but we don't really want to make getting cozy with OpenPGP a pre-requisite of building rpms, rpm itself is plenty enough of learning curve for a first-timer. We should have a no-brainer procedure to set up an rpmbuild-specific signing key and configure rpmbuild to use it.

For this, we need to determine whether signing is configured at the start of an rpmbuild and either stop the build with a single command instruction to configure signing, or just automatically set it up on first run. So I guess the first step is to create a script that does the setup, with an option to use either users existing key setup or fully rpm-specific keys. At least pre-existing keys will need another option for gpg/sq.

At least for a fully automated rpmbuild specific key, it's best buried inside ~/.config/rpm and for that makes this at least partially depend on #3467.