Add a new 'why' argument to kdb_enter(), and a set of constants to use

Add a new 'why' argument to kdb_enter(), and a set of constants to use
for that argument.  This will allow DDB to detect the broad category of
reason why the debugger has been entered, which it can use for the
purposes of deciding which DDB script to run.

Assign approximate why values to all current consumers of the
kdb_enter() interface.