make SW_WATCHDOG dynamic

make SW_WATCHDOG dynamic

Enable the hardclock-based watchdog previously conditional on the
SW_WATCHDOG option whenever hardware watchdogs are not found, and
watchdogd attempts to enable the watchdog. The SW_WATCHDOG option
still causes the sofware watchdog to be enabled even if there is a
hardware watchdog. This does not change the other software-based
watchdog enabled by the --softtimeout option to watchdogd.

Note that the code to reprime the watchdog during kernel core dumps is
no longer conditional on SW_WATCHDOG. I think this was previously a bug.

Reviewed by:	imp alfred bjk
MFC after:	1 week
Relnotes:	yes
Differential Revision:	https://reviews.freebsd.org/D13713