services: Add Shepherd 'configuration' action to various services.

services: Add Shepherd 'configuration' action to various services.

* gnu/services/avahi.scm (avahi-shepherd-service): Add 'actions' field.
* gnu/services/base.scm (nscd-actions): Add call to
'shepherd-configuration-action'.
* gnu/services/desktop.scm (upower-shepherd-service): Add 'actions'
field.
(elogind-shepherd-service): Likewise.
* gnu/services/dict.scm (dicod-shepherd-service): Likewise.
* gnu/services/networking.scm (openntpd-shepherd-service): Likewise.
(tor-shepherd-service): Likewise.
* gnu/services/ssh.scm (openssh-shepherd-service): Likewise.
* gnu/services/web.scm (nginx-shepherd-service): Likewise.
* gnu/services/xorg.scm (gdm-shepherd-service): Likewise.
* gnu/tests/base.scm (run-basic-test)["nscd configuration action"]: New
test.
* doc/guix.texi (Services): Document it.