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

Author: anarazel

contrib/file_fdw/file_fdw.c

@@ -622,7 +622,7 @@ fileExplainForeignScan(ForeignScanState *node, ExplainState *es)
 
 		if (!is_program &&
 			stat(filename, &stat_buf) == 0)
-			ExplainPropertyInteger("Foreign File Size",
+			ExplainPropertyInteger("Foreign File Size", "b",
 								   (int64) stat_buf.st_size, es);
 	}
 }