Add support for building bootstrap gcc with libc/kernel headers (--with-headers)...

Add support for building bootstrap gcc with libc/kernel headers (--with-headers), fix cleaning of categories/package.keywords files, add generic support for future --with-/--without- options, better detect when a package needs to be re-emerged or can be safely skipped, and touchup code in general.
Package-Manager: portage-2.0.53_rc7