MFC r301522 (by bz)

Implement a show panic command to DDB which will helpfully print the
panic string again if set, in case it scrolled out of the active
window. This avoids having to remember the symbol name.

Also add a show callout <addr> command to DDB in order to inspect
some struct callout fields in case of panics in the callout code.
This may help to see if there was memory corruption or to further
ease debugging problems.

No objection by: bz