mask: support auto filing of last-rite bug & PMASKED bugs

And my newest hat trick, auto filing of last-rite bugs. I was getting fed up with all the `lr-file-bug` calls during the EAPI=6 cleanup, passing all the args correctly in the form, and then copying everything back to `pkgdev mask` invocation. So since I'm that comfortable with filing bugs using `pkgdev bugs`, why not import it into `pkgdev mask`? 1. File a new bug, CC treecleaners, auto assign to all maintainers, add blocks on reference bugs. 2. Add PMASKED keyword to all referenced bugs. 3. Continue as usual.