services: Add a default value to various service types.

* gnu/services/admin.scm (rottlog-service-type)[default-value]: New
field.
* gnu/services/base.scm (guix-service-type)[default-value]: New field.
(guix-publish-service-type)[default-value]: New field.
* gnu/services/cups.scm (cups-service-type)[default-value]: New field.
* gnu/services/dict.scm (dicod-service-type)[default-value]: New field.
* gnu/services/mcron.scm (mcron-service-type)[default-value]: New field.
* gnu/services/networking.scm (<tor-configuration>)[config-file]: Add
default value.
(tor-service-type)[default-value]: New field.
(<bitlbee-configuration>)[interface, port, extra-settings]: Add default
values.
(bitlbee-service-type)[default-value]: New field.
(wpa-supplicant-service-type)[default-value]: New field.
(tlp-service-type)[default-value]: New field.
(openssh-service-type)[default-value]: New field.
* doc/guix.texi (Base Services, Log Rotation)
(Networking Services, Printing Services):
(Power management Services): Adjust examples accordingly.