proc.io.* - tweak cache logic
proc.io.* - tweak cache logic

Document which Linux kernel config parameters need to be set before
proc/<pid>/io entries appear.  And as a result of running a kernel
without this, improve the cache flag logic so bogus values are not
reported for all instances except the first ... sigh.

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