Fix issues with photo and voltage stimulus.

Photostimulus shutter would not close at end of recording, and voltage
stimulus protocols would not start on some rigs at start of recording.
Small changes to RecADCOnly.pas were sufficient to address these.
Fix bug in stimulus_issues which erroneously caused photo stimulus start.

The change made in stimulus_issues to address stimulus protocols not
starting on some rigs was found to erroneously toggle the photostimulus
protocol as well.  This change makes sure only the voltage stimulus is
toggled.