Minor cleanups to mmv_stats header file.
Minor cleanups to mmv_stats header file.

- Use extern "C" in case client is a C++ program.
- Declare functions as "extern".
- Clarify some comments.

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