making PANIC_REBOOT_WAIT_TIME a tunable

Hi all,

It seems that PANIC_REBOOT_WAIT_TIME has been a compile-time setting forever;
and I can't see any reason for this, but I assume there was one... at some
point in the distant past.

The attached patch makes it a loader tunable and sysctl.  My reason for wanting
this is to make EC2 images reboot faster after a panic (not that it happens
very often, of course) -- there's no point waiting for a key press at the
console because the EC2 console is output-only.

Any objections?

-- 
Colin Percival
Security Officer Emeritus, FreeBSD | The power to serve
Founder, Tarsnap | www.tarsnap.com | Online backups for the truly paranoid