UART: how to check pending transmit count

[rbarris]

I trawled the ESP-IDF uart.c / uart.h for some time and could not figure out what the right way is, to get a true/false answer to the question "are there any remaining untransmitted characters in the tx buffer or the UART hardware FIFO".

I explicitly do not want a blocking send or flush call; I want a test to find out whether the transmit queue has drained to completion.