Fix chunk allocation/deallocation bugs.

Fix chunk_alloc_dss() to zero memory when requested.

Fix chunk_dealloc() to avoid chunk_dealloc_mmap() for dss-allocated
memory.

Fix huge_palloc() to always junk fill when requested.

Improve chunk_recycle() to report that memory is zeroed as a side effect
of pages_purge().

src/chunk_dss.c

@@ -89,6 +89,10 @@ chunk_alloc_dss(size_t size, size_t alignment, bool *zero)
 				malloc_mutex_unlock(&dss_mtx);
 				if (cpad_size != 0)
 					chunk_dealloc(cpad, cpad_size, true);
+				if (*zero) {
+					VALGRIND_MAKE_MEM_UNDEFINED(ret, size);
+					memset(ret, 0, size);
+				}
 				return (ret);
 			}
 		} while (dss_prev != (void *)-1);