Add userland access to at91 gpio functionality via ioctl calls.  Also,
add the ability for userland to be notified of changes on gpio pins via
a select(2)/read(2) interface.

Change the interrupt handler from filtered to threaded.

Because of the uiomove() calls in the new interface, change locking from
standard mutex to sx.

Add / restore the at91_gpio_high_z() function.

Reviewed by:	imp (long ago)
Disable this; it's a local option that I haven't yet committed to -HEAD.