Add Serial_::availableForWrite()

Fix #149

Add Uart::availableForWrite()

Correct SerialUSB return value

Port of https://github.com/arduino/Arduino/pull/4996

Previous 0 was always returned.

This enables WDT, so sketches do not work.

Port of
https://github.com/arduino/Arduino/commit/6a6ed3d10ad1470283d7771906ce81
ad97fa06f0

Merge branch 'archive_file_path-backwards-compatibility' of https://github.com/sandeepmistry/ArduinoCore-samd

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);

Port of https://github.com/arduino/Arduino/pull/4737