queue.scm: Split postfix-service-type into {local,internet}-postfix-service-type...Also add those to `Makefile` as separate targets.
BONUS: better naming of Guix targets in `Makefile`. Instead of
`all-{packages,services}`, it is now `guix-{packages,services}`,
as there are other packages being built that do not belong to
Guix, and maybe more services in the future.