gnu: Add inputattach service.

Add a service that runs inputattach as a daemon to translate events from
serial ports.

* gnu/services/desktop.scm (<inputattach-configuration>): New record type.
* gnu/services/desktop.scm (inputattach-service-type): New service type.
* doc/guix.texi (Miscellaneous Services): Add inputattach Service
  subsubheading.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>