conditional enter to DDB on busy buffers

[Richard Nyberg <rnyberg@xxxxxxxx>]

Since I don't have a serial console I don't want my computer
to risk entering the debugger at reboot if it cannot sync its buffers.
The patch at the URL below checks the "debugger_on_panic" sysctl to
conditionalize use of the debugger.

http://people.su.se/~rnyberg/dfly/busy_buffer_ddb.diff

Happy hacking!
        -Richard