Log: crmd,libcrmcommon: improve log messages and comments

Most importantly, attrd-related messages were inconsistent and sometimes
misleading (purge and refresh requests could be logged as null name-value
updates). E.g. before:

debug: attrd_update_delegate:    Sent update: (null)=(null) for localhost

and after:

debug: attrd_update_delegate:    Asked attrd to refresh localhost: OK (0)