man/man3/pmgetoptions.3: document secret env vars
man/man3/pmgetoptions.3: document secret env vars

After scratching my head over how command line args get from
pcp-summary to libpcp, I stumbled across this treasure trove
of undocumented environment variables hiding below pmGetOptions().

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