Add 'unit' parameter to ExplainProperty{Integer,Float}.
This allows to deduplicate some existing code, but mainly avoids some
duplication in upcoming commits.
In passing, fix variable names indicating wrong unit (seconds instead
of ms).
Author: Andres Freund
Discussion: https://postgr.es/m/
20180314002740.cah3mdsonz5mxney@alap3.anarazel.de