services: Export *-service-type and *-configuration.

services: Export *-service-type and *-configuration.

This allows users to use 'modify-services' and similar constructs for
all these service types.

* gnu/services/avahi.scm: export avahi-configuration.
* gnu/services/base.scm: export gpm-configuration and
rngd-configuration.
* gnu/services/databases.scm: export *-service-type and *-configuration.
* gnu/services/dbus.scm: export dbus-configuration.
* gnu/services/dict.scm: export dicod-service-type.
* gnu/services/lirc.scm: export lirc-configuration and
lirc-service-type.
* gnu/services/mail.scm: export dovecot-service-type.
* gnu/services/web.scm: export nginx-configuration and
nginx-service-type.
* gnu/services/xorg.scm: export screen-locker and screen-locker?.
* gnu/services/ssh.scm: export lsh-configuration and lsh-service-type.
* gnu/services/desktop.scm: export *-service, *-service-type
and *-configuration.
* gnu/services/networking.scm: export *-configuration
and *-service-type.

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