Fixed PWM frequency glitch

Port of https://github.com/arduino/Arduino/pull/4718 to SAMD core.

Use TX buffer for Wire slave writes

Enable digitalRead on output pins, only enable pull-up in digitalWrite if pin is not output

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