Fix pmda_proc (linux) to allow PIDs >= 1000000
Fix pmda_proc (linux) to allow PIDs >= 1000000

Previously, when pid's greater than 999999 were fetched, the output
became mangaled, potentially crashing some client tools.  Adjust the
size of 'buf' comparisons accordingly.

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