In the troff output, this doesn't seem to make any difference. But in the
html output, the whitespace is sometimes preserved, creating an additional
gap before the following content. Drop it everywhere to avoid this.
We have this in a similar fashion for the other APIs libsystemd
provides. Add the same for sd-varlink. There isn't too much on it for
now, but at least it's a start.
Also link it up everywhere.
Before libsystemd-daemon, libsystemd-journal, libsystemd-id128, etc., were
merged into libsystemd, it was enough to have individual man pages for them.
But they have been delivered as one thing for many years, so it's better to
have a landing page for libsystemd. It mostly directs to individual pages
anyway.
