sysctls in kern_shutdown: add twin tunables

http://people.freebsd.org/~avg/kern_shutdown-tunables.diff

The above patch adds twin tunables for the following (R/W) sysctls:
- debug.debugger_on_panic
- debug.trace_on_panic
- kern.sync_on_panic

This seems useful to me, but I am not sure if I am not missing something
important.  E.g. security-wise.
It seems that I am not paranoid enough often times.

-- 
Andriy Gapon