Let's systematically make sure that we link up the D-Bus interfaces from
the daemon man pages once in prose and once in short form at the bottom
("See Also"), for all daemons.
Also, add reverse links at the bottom of the D-Bus API docs.
Fixes: #34996
This commit introduces a build-time option to enable/disable sysupdated
separately from sysupdate. 'auto' translated to enabled by default in
developer builds.
