services: inputattach: Add 'baud-rate' parameter.

* gnu/services/desktop.scm (inputattach-configuration): Add baud-rate field.
  (inputattach-shepherd-service): Add baud-rate to parameters when specified.
* doc/guix.texi (Miscellaneous Services): [inputattach Service] Document
  baud-rate parameter.

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