Improve geometric BVL display calibration routines.

Wait for keyboard input on all connected keyboards.
Add support to select stereoMode for display calibration,
e.g., mode 4 for dual-display co-calibration.

Improvements mostly contributed by Bas Rokers.
PRxxxinit: Don't flush serial port buffers on write.

Add DontFlushOnWrite=1 option to serial port open for the PRxxx
photometer toolboxes. At least on PR-Photometer hangs in the
flush on Linux and we don't need flushing for those functions
anyway, so just avoid it for all of them on all os'es.

Should fix problem reported in forum message #15652.