[FIXED JENKINS-22672] Make /threadDump easier to use.

[FIXED JENKINS-22688] Made graph font explicit and configurable

[FIXED JENKINS-22689] Show load statistics for master node

Instead, use a distance of 0.5x the width/height of the label.

This improves appearance in combination with JENKINS-22688 for
different font sizes.

Debian postinst: check for present user/group before adding them

pending was null as soon as task started running, so it was possible that another thread submits the task and have that executed while the first one was still running

This is in preparation of resolving dependencies within "List<URL> all"

Fix issue where restart would not properly display username if user was not loaded.

In git commit 7216aab0 the check for present user was removed
and therefore upgrading the Debian package on existing Jenkins
installations fails. Also the newly added code for adding a
configurable Jenkins group is missing a check for a possibly
already present Jenkins group.

Closes: JENKINS-22710