postfix pmda: run as the postfix user, not the pcp user
postfix pmda: run as the postfix user, not the pcp user

The Postfix PMDA uses qshape(1) to gather postfix queue stats,
by reading files the various postfix queue directories.  All
of the postfix queues are only readable by the postfix user or
root.

Initial problem report and testing brought to you by the good
folks at messagemedia.com.au.

aeg-aeg/pcpfans,adfernandes/pcp,aeg-aeg/pcpfans,andyvand/cygpcpfans,prasincs/pcp,edwardt/pcp,edwardt/pcp,edwardt/pcp,tjanez/pcp,edwardt/pcp,andyvand/cygpcpfans,mbaldessari/pcp,tjanez/pcp,aeg-aeg/pcpfans,edwardt/pcp,wuliming/pcp,prasincs/pcp,prasincs/pcp,wuliming/pcp,wuliming/pcp,edwardt/pcp,tjanez/pcp,edwardt/pcp,tjanez/pcp,wuliming/pcp,mbaldessari/pcp,adfernandes/pcp,adfernandes/pcp,mbaldessari/pcp,mbaldessari/pcp,andyvand/cygpcpfans,aeg-aeg/pcpfans,adfernandes/pcp,adfernandes/pcp,aeg-aeg/pcpfans,wuliming/pcp,prasincs/pcp,aeg-aeg/pcpfans,adfernandes/pcp,tjanez/pcp,prasincs/pcp,aeg-aeg/pcpfans,tjanez/pcp,prasincs/pcp,mbaldessari/pcp,adfernandes/pcp,wuliming/pcp,prasincs/pcp,aeg-aeg/pcpfans,andyvand/cygpcpfans,edwardt/pcp,mbaldessari/pcp,adfernandes/pcp,andyvand/cygpcpfans,wuliming/pcp,tjanez/pcp,andyvand/cygpcpfans,prasincs/pcp,wuliming/pcp,andyvand/cygpcpfans,andyvand/cygpcpfans,tjanez/pcp