Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

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.

Make pswitch_intr() returns interrupt handling status.

Catch up with bus_setup_intr() modification and garbage collect a
reference to INTR_FAST.

/* -> /*- for license, minor formatting changes

Make this compile: add sys/module.h and KDBify.

Oops. Include opt_ddb.h.

Add a driver that attaches to the gpio node of macio and allows you to enter
DDB when the interrupt button (aka the "programmer's switch") is pressed.

This isn't unfortunately an NMI, but it's a handy way to get into DDB
quickly if needed.