libpcp: PCP_DERIVED_CONFIG=="" disables loading derived metrics
libpcp: PCP_DERIVED_CONFIG=="" disables loading derived metrics

Also add a comment explaining the three use-cases:

If PCP_DERIVED_CONFIG is NOT set, then by default we load global
derived configs from the directory $PCP_VAR_DIR/config/derived.

If PCP_DERIVED_CONFIG is set to a zero length string, then don't
load any derived metrics definitions.

Else if PCP_DERIVED_CONFIG is set then load user-defined derived
metrics from one or more files or directories separated by ':'.

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