services: Add agetty service.

* gnu/services/base.scm (<agetty-configuration>): New record type.
(agetty-shepherd-service, agetty-service): New procedures.
(agetty-service-type): New variable.
* doc/guix.texi (Base Services): Document it.
[mingetty-configuration],[kmscon-configuration]: Specify the types of
supported consoles.