PrintPhotoReceptors, FillInPhotoreceptors: Bug fixes and restore Octave compatibility.

fields() doesn't exist in Octave 3.6.x, but fieldnames() does and does
the same thing.
Use proper short-circuit boolean operators.
Replace obsolete isstr() by ischar().
Fix loop variable bugs.
Linux XLib locking: The horror continues. Fix PsychHID XInitThreads()

Remaining use of XInitThreads in PsychHID can trigger crashes in
Screen() shutdown, due to mixup of non-threading and threading
x-connections in Screen vs. PsychHID. Only happened on Yaroslavs setup.

-> Make use of XInitThreads() in both Screen and PsychHID conditional
on a defined environment variable PSYCH_XINITTHREADS. By default it is
not used, as we have our own locking for Screen() and don't need it in
PsychHID due to it using dedicated x-connections for its threads (main
thread vs. kbqueue thread).
-> The environment variable is for debugging only, or in the faint hope
that it might fix crashes with the FOSS mesa intel/nouveau..
gfx-drivers caused by DRI2's racy way of sending drawable invalidate
events to clients. We'll see if it helps in those cases or not. Our own
locking definitely doesn't work reliably there on the Intel driver.
Rebuild Screen and PsychHID for Linux with latest XLib threading fixes.

All 32/64-Bit Octave/Matlab for Intel cpus.
No rebuild for 32-Bit ARM, the 3 people that use it will have to suffer
a bit longer.
Eyelink: Improve error handling in Eyelink('Initialize').

Provide meaningful error message with abort if open_eyelink_system()
fails, instead of silent failure and confusing status return codes.
-> Minor problem, no need to rebuild Eyelink mex file. Will be folded
into release at next real rebuild.
New demo TurnTableDemo - Free your inner DJ!

A demo which uses PsychPortAudio's functions to simulate a good'ol turn
table for some recreational scratching and other auditory fun
activities.

Work in progress - This is a backup commit of a basic working state.
TurnTableDemo: Number zero does not map well - exclude it.
TurnTableDemo: Refine key mappings.

Right ALT doesn't work well on some setups, so use
Right SHIFT instead to stop looping sounds and drop
the old mute function of that right SHIFT key.

Minor cleanups.