pcp-dstat: exit with code zero when archive end reached
pcp-dstat: exit with code zero when archive end reached

This fixes up a failure on recently created test qa/1207.

adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp