ModbusMaster fixes

[kmihaylov]

This PR introduces the following fixes to ModbusMaster:
0. ModbusMaster is patched to use single _serial->write() to send the whole message instead of 'for loop'.

1. ModbusMaster is now patched so as to send single NUL character after the message in order to ensure TX line release after the whole message is sent.
2. Delays in the generic sample are commented out, but left for reference (the example works well without the delays).
3. In mbLoop() the variable result was used instead of mbResult when reporting the error code. result is removed and the proper mbResult is used.
4. In mbLogReceive() debug message of the RX ADU is added.
5. In mbLogTransmit ADU is changed to "TX ADU" to clarify the message.

[mikee47]

Looks fine! Thanks.