Replace chunk stats code that was missing locking; this fixes a race
condition that could corrupt chunk statistics.
Converting malloc_stats_print() to use mallctl*().
Add a missing semicolon in th DSS code.
Convert malloc_tcache_flush() to a mallctl.
Convert malloc_swap_enable() to a set of mallctl's.
Use optional zeroing in arena_chunk_alloc() to avoid needless zeroing of
chunks. This is particularly important in the context of swapfile and
DSS allocation, since a long-lived application may commonly recycle
chunks.
