services: Add getmail.

services: Add getmail.

Getmail is a mail retriever written in Python, this commit adds a service-type
to run getmail. I'm looking at this, as it's a convinient way of getting
mailing list messages in to Patchwork.

I initially tried putting this in the (gnu services mail) module, but due to
also trying to use the define-configuration pattern, it conflicted with the
dovecot service.

* gnu/services/getmail.scm: New file.
* gnu/local.mk: Add it.
* gnu/tests/mail.scm (%getmail-os, %test-getmail): New variables.
(run-getmail-test): New procedure.