Add userland access to at91 gpio functionality via ioctl calls. Also,

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)