Half duplex support

[peteruithoven]

Adds a parameter to the Software Serial library to enable a half-duplex mode. This allows using 1 line for RX and TX.
It does this by setting the transmit pin by default to input, with the pull-up set. When transmitting, the pin temporarily switches to an output until the byte is sent, then flips back to input. When a module
is receiving it should not be able to transmit, and vice-versa.
Based on the following fork: https://github.com/nickstedman/SoftwareSerialWithHalfDuplex

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[peteruithoven]

It's been 4 years, I hope someone else can pick this up.