PR-Photometer toolboxes: Add 'Lenient' option to serial port open.

Add the 'Lenient' keyword so IOPort accepts some non-critical
failures during serial-port connection open, e.g., the failure
to enable low-latency mode on a serial link, without aborting
with error.

Some serial port drivers don't support such settings, so failure
would be guaranteed, e.g., the PR-XXX photometer setup on Linux.

Should fix issue from Psychtoolbox forum message #15535.

Modified PR650/655/670/705init.m