build-sys: add sysusers.d/systemd.conf to CLEANFILES

The sysusers.d/systemd.conf configuration file was originally introduced in
commit 1b99214789101, but it was not marked for cleanup.  This caused distcheck
to complain about the file not being removed by distcleam.

Tested: Successfully ran "make distcheck" with this patchset.
build-sys: fix indentation in make-man-rules generated output

This fixes the footer where 7 spaces were used on the first line of EXTRA_DIST,
but a Tab was clearly intended.