Do not warn about update/ when we're updating the updater

This is in preparation for running the just-updated updater from the
.jar files *in* update/.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
When updating the updater, do not delete currently used .jar files

In the old Fiji days, we used to overwrite the .jar files when updating
the updater, but that only really worked because the file names did not
have version numbers in them. Now they do, which means that files that
are currently in use are deleted (never mind that this is not possible
on Windows).

So let's change the strategy and *not* overwrite those files, but instead
run the updater in-place from the files in update/ that will be moved
into their final destinations upon restart.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
After updating the updater, reset the indicator that we are doing that

When we're updating the updater, we must not complain about update/
being present. To know when not to complain, we set a system
property. After updating, that system property needs to be cleared
so we know that we can start complaining about update/ being present
all we want again.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>