Add kern.reboot_wait_time sysctl

Historic FreeBSD behaviour (dating back to 1994-04-02) when rebooting
is to print "Rebooting..." and then
/* wait 1 sec for printf's to complete and be read */

Prior to April 1994, there was a 100 ms delay (added 1993-11-12).

Since (a) most users will already be aware that the system is rebooting
and do not need to take time to read an additional message to that
effect, and (b) most FreeBSD systems don't have anyone actively looking
at the console anyway, this delay no longer serves much purpose.

This commit adds a kern.reboot_wait_time sysctl which defaults to 0;
historic behaviour can be regained by setting it to 1.

Reviewed by: imp
Relnotes: FreeBSD now reboots faster; to restore the traditional

		wait after printing "Rebooting..." to the console, set
		kern.reboot_wait_time=1 (or more).

Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D35796