* gnu/home/services/upnp.scm: New file.
* gnu/local.mk: Register it.
* gnu/services/upnp.scm: Export readymedia-activation and
readymedia-shepherd-service.
(<readymedia-configuration>)[home-service?]: New field.
[cache-directory]: Adjust value depending on 'for-home?'.
[log-directory]: Ditto.
(readymedia-shepherd-service): Adjust 'requirement' and 'start' according to
'home-service?'.
(readymedia-activation): Adjust creating 'media-directories' with permissions
according to 'home-service?'.
* gnu/tests/upnp.scm (%readymedia-configuration-test): Configure port with
%readymedia-default-port.
* doc/guix.texi (Miscellaneous Home Services): Document Readymedia Service.
(Miscellaneous Services): Add cross-reference.
Change-Id: I5c48595d84a815d98e03c7f68a716f048903720c
Signed-off-by: Ludovic Courtès <ludo@gnu.org>