panic: Optionally, trace secondary panics

To diagnose and fix secondary panics, it is useful to have a stack trace.
When panic tracing is enabled, optionally trace secondary panics as well.

The option is configured with the tunable/sysctl debug.trace_all_panics.

(The original concern that inspired only tracing the primary panic was
likely that the secondary trace may scroll the original panic message or trace
off the screen. This is less of a concern for serial consoles with logging.
Not everything has a serial console, though, so the behavior is optional.)

Discussed with: jhb
Sponsored by: Dell EMC Isilon