This is a followup to 193f547ca3, which
inadvertently pulled in (shepherd support) on the host side.
* gnu/home/services/messaging.scm (home-znc-services): Change 'command' and
'log-file' to gexps. Add 'modules' field to 'shepherd-service'.
* gnu/home/services/messaging.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* po/guix/POTFILES.in: Likewise.
* doc/guix.texi (Messaging Home Services): Document it in new subsection.