seperate out call_prog's overloading of $retval into two variables.
this was causing expressions which expected a numeric $retval to succeed
where they shouldn't have.
e.g. a shepherd.log from a user indicated shepherd was accepting the output from a failed plugin even though it shouldn't have:
SHEPHERD: Using postprocessor: augment_timezone
SHEPHERD: Excuting command: /home/mythtv/.shepherd/postprocessors/augment_timezone/augment_timezone --region 73 --channels_file /home/mythtv/.shepherd/channels.conf --output /home/mythtv/.shepherd/postprocessors/augment_timezone/output.xmltv --days 7 /home/mythtv/.shepherd/reconcilers/reconciler_mk2/output.xmltv
:::::: Output from augment_timezone
: adjust_timezone v0.04
: Not a SCALAR reference at /home/mythtv/.shepherd/postprocessors/augment_timezone/augment_timezone line 110.
:::::: End output from augment_timezone
SHEPHERD: Warning: plugin 'augment_timezone' output file '/home/mythtv/.shepherd/postprocessors/augment_timezone/output.xmltv' does not exist
SHEPHERD: accepting output from postprocessor augment_timezone, feeding it into next stage
whereas it should have rejected it up-front.
resolves bugs introduced in [401], [406] and [410].
