Sync prompt timeout code with sysupgrade watchdog

Sync prompt timeout code with sysupgrade watchdog

Simplify code and employ the same technique in both places.
The "prompt timeout" hits when on non-interactive installations or upgrades:

	Welcome to the OpenBSD/amd64 6.9 installation program.
	Starting non-interactive mode in 5 seconds...
	(I)nstall, (U)pgrade, (A)utoinstall or (S)hell?
	Performing non-interactive upgrade...

The existing code uses a regular background job and does quirky file
descriptor fiddling just to avoid job control messages when starting the
timeout in the background.

It also does `set +m' a second time for no reason and hardodes the number of
seconds in multiple places.

Do better by using a co-process just like sysupgrade's watchdog that reboots
when the upgrade did not finish in time.

reads good to aja