By default RTS/DTR seem to be True (-> 0 since low active). Most ESP32
boards have a pair of transistors to keep Reset/strapping GPIO as-is
when both RTS/DTR are either True _or_ False.
See: https://raw.githubusercontent.com/nodemcu/nodemcu-devkit/master/Documents/NODEMCU_DEVKIT_SCH_BIG.png

If RTS/DTR are connected to Reset/strapping GPIO directly we'd prefer to have
RTS/DTR False (-> 1 since low active). In theory, we could set it to False
always, since the above mentioned logic makes sure nothing happens if both
signals are False.

However, on some platforms (e.g. Linux) setting them to False leads to toggling
when opening which causes a reset.

This introduces a new config "low_rts_dtr" which allows to set the
signals low if required by the board.