services: connman: Rework service.

* gnu/services/networking.scm (connman-service): Remove.
(<connman-configuration>): New record specifying the package
to be used (connman) and whether vpn plugin shall be
disabled (disable-vpn?).
(connman-configuration): New exported variable.
(connman-configuration?): New exported variable.
(connman-service-type): Export it.

* doc/guix.texi (Networking Services): Adjust accordingly.

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