PsychScriptingGlue: Only ever try to assign one subfunction argument.

If first passed arg is a valid subfunction, skip trying to interpret
2nd arg as subfunction as well. That would be non-sensical and
additionally causes the subfunction table dispatch to interpret any
question mark at the end as a request for help, so e.g.,

Eyelink('Message', 'What's up?'); would try to parse 'What's up?'
as a 2nd subfunction name and then, seeing the '?' at the end,
print function help texts instead of executing 'Message' with
'What's up?' as text string argument.
PsychScriptingGlue: Fix global state bleeding between mex files.

Make prhs/plhsGLUE et al. static so their state can't bleed from
one PTB mex file to the other. This was the case when Screen()
was calling DataPixx() from within its post-swap hooks to do
timestamping with DataPixx. Datapixx was globbering the state
of Screen. This was the reason for the errors in VBLSyncTest.m
when Datapixx + Octave was used. The error didn't happen on Matlab
because there we compile mex files to only export mexFunction()
but nothing else into the process symbol space.

-> Fixed.
-> Screen for 64-Bit Octave on Linux rebuilt and tested.
-> VBLSyncTest: Remove all workarounds.