Move code into new stop_watchdog()

Move code into new stop_watchdog()

We have {reset,start}_watchdog() which are only used in unattended upgrade
code, but stopping the background timer is done inline for all upgrades,
incl. interactive ones.

Relocate it out of the very end of do_upgrade() right after its only caller
and limit it to unattended upgrades to match where/how the timer is started.

OK afresh1