services: Add 'dropbear-service'.

* gnu/services/ssh.scm (<dropbear-configuration>): New record type.
(dropbear-activation, dropbear-shepherd-service, dropbear-service): New
procedures.
(dropbear-service-type): New variable.
* doc/guix.texi (Networking Services): Document it.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>