Add arena-specific and selective dss allocation.

Add the "arenas.extend" mallctl, so that it is possible to create new
arenas that are outside the set that jemalloc automatically multiplexes
threads onto.

Add the ALLOCM_ARENA() flag for {,r,d}allocm(), so that it is possible
to explicitly allocate from a particular arena.

Add the "opt.dss" mallctl, which controls the default precedence of dss
allocation relative to mmap allocation.

Add the "arena.<i>.dss" mallctl, which makes it possible to set the
default dss precedence on a per arena or global basis.

Add the "arena.<i>.purge" mallctl, which obsoletes "arenas.purge".

Add the "stats.arenas.<i>.dss" mallctl.

include/jemalloc/internal/ctl.h

@@ -33,6 +33,7 @@ struct ctl_indexed_node_s {
 struct ctl_arena_stats_s {
 	bool			initialized;
 	unsigned		nthreads;
+	const char		*dss;
 	size_t			pactive;
 	size_t			pdirty;
 	arena_stats_t		astats;
@@ -61,6 +62,7 @@ struct ctl_stats_s {
 		uint64_t	nmalloc;	/* huge_nmalloc */
 		uint64_t	ndalloc;	/* huge_ndalloc */
 	} huge;
+	unsigned		narenas;
 	ctl_arena_stats_t	*arenas;	/* (narenas + 1) elements. */
 };