build: *really* install tmpfiles.d/systemd-remote.conf when necessary (#6061)

This fixes commit 5e354b22 which was an attempt to avoid installation
of tmpfiles.d/systemd-remote.conf when it was not needed (ie HAVE_REMOTE=false).

Before this fix and with the autotool build, systemd-remote.conf was
distributed and also installed (although it was empty) even though
HAVE_REMOTE=false.

That's what happens when doing last second changes without retesting...

While at it, update tmpfiles.d/.gitignore (var.conf was missing as well
since commit a083537e).