* gnu/home/services/syncthing.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* gnu/services/syncthing.scm (<syncthing-configuration>)[home-service?]:
New field.
Adjust 'provision' and 'requirement' depending on 'home-service?', and
likewise for #:user and #:group.
Use 'filter' + 'negate' instead of 'remove'.
* doc/guix.texi (Networking Services): Add note and cross-reference to
"Networking Home Services".
(Networking Home Services): New node.
