Change digitalPinToInterrupt to return pin, and use g_APinDescription[pin].ulExtInt to map pin inside attachInterrupt/detachInterrupt

This brings things inline with the atachInterrupt documentation for
usage:
attachInterrupt(digitalPinToInterrupt(pin), ISR, mode);

Fix #124

Fix #126

Port of @tico-tico’s change in
https://github.com/tico-tico/Arduino/commit/a7454b6b5c59187b95c4224aad87
bb01faa06e85 to SAMD core.

This prevents accidental overwrites of the bootloader if a NULL
pointer is used for writing by mistake.

The pin mapping is still subject to change, better to not distribute
files that are not final.

USBHost used to have an init() method that now is incorporated in
Init(). The leftover init() calls in the library were calling
the init() function in the variant files instead (that has nothing
to do with the USBHost library).

Removing the calls to init() fix it.

Fix #115

0x00 CTRL-@ through 0x1F CTRL-_

Also return ASCII codes for keys: ESCAPE, DELETE, TAB, and BACKSPACE keys.
Return correct code for ENTER and keypad ENTER keys.

https://github.com/felis/USB_Host_Shield_2.0/pull/185

Affects all devices that do IN transfers.