pcp

Dynamic names rework after derived metrics added.

The PMID for derived metrics and for the root of a dynamic subtree in
the PMNS both use a domain of DYNAMIC_PMID ... tighten up tests to be
sure that we're not confused and consistently using the following tests:

dynamic root of pmns
pmid_domain(pmid) == DYNAMIC_PMID && pmid_item(pmid) == 0
// pmid_domain(cluster) is the "real" domain number for the
// responsible PMDA

derived metric
pmid_domain(pmid) == DYNAMIC_PMID && pmid_item(pmid) != 0
// pmid_domain(cluster) will be 0