Add ability to dump stacktraces on kernel panics when DDB is compiled into the kernel.  By default this is turned off since otherwise it could scroll valuable panic messages off of the screen.  This option can be turned on by the DDB_TRACE kernel option as well as the debug.trace_on_panic sysctl.
Add ability to dump stacktraces on kernel panics when DDB is compiled into
the kernel.  By default this is turned off since otherwise it could scroll
valuable panic messages off of the screen.  This option can be turned on
by the DDB_TRACE kernel option as well as the debug.trace_on_panic sysctl.

Also, fix the DDB_UNATTENDED option to use its own header instead of
abusing opt_ddb.h.  This way turning that one option on or off doesn't
force you to recompile all of ddb.

Requested by:	many (1), bde (2*)

* - I know bde prefers !abusing option headers in general but can't
    remember if he as brought up this specific case.

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase