Add the stats.retained and stats.arenas.<i>.retained statistics.

This resolves #367.

include/jemalloc/internal/stats.h

@@ -102,6 +102,14 @@ struct arena_stats_s {
 	/* Number of bytes currently mapped. */
 	size_t		mapped;
 
+	/*
+	 * Number of bytes currently retained as a side effect of munmap() being
+	 * disabled/bypassed.  Retained bytes are technically mapped (though
+	 * always decommitted or purged), but they are excluded from the mapped
+	 * statistic (above).
+	 */
+	size_t		retained;
+
 	/*
 	 * Total number of purge sweeps, total number of madvise calls made,
 	 * and total pages purged in order to keep dirty unused memory under