When an upgrade is running, prevent requests for admin/upgrade.php
from immediately starting up a second unwanted upgrade by inserting
a record in the config table.  Nothing is done on fresh installs,
because the config table won't exist.

Change-Id: Ib82505d04fcb90b0ccbb461acd0d0b4646460498
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>