Disable munmap() if it causes VM map holes.

Add a configure test to determine whether common mmap()/munmap()
patterns cause VM map holes, and only use munmap() to discard unused
chunks if the problem does not exist.

Unify the chunk caching for mmap and dss.

Fix options processing to limit lg_chunk to be large enough that
redzones will always fit.

src/chunk_mmap.c

@@ -178,11 +178,14 @@ chunk_alloc_mmap(size_t size, size_t alignment)
 	return (ret);
 }
 
-void
+bool
 chunk_dealloc_mmap(void *chunk, size_t size)
 {
 
-	pages_unmap(chunk, size);
+	if (config_munmap)
+		pages_unmap(chunk, size);
+
+	return (config_munmap == false);
 }
 
 bool