alloc: revert "zero all new memory from allocations" (#959)

Commit d046525d made my_alloc() calloc every fresh allocation and made
expand_item_list() memset the freshly grown tail, to hand out predictably
zeroed memory.  But that forces the kernel to back pages callers never
touch: each per-directory file_list pre-allocates a FLIST_START-entry
(32768) pointer array -- 256KB -- and calloc now zeroes the whole array
even for an empty directory.  With incremental recursion over many
directories the resident set explodes; 80000 empty dirs went from ~336MB
to ~10.8GB.

Restore the pre-d046525d malloc/calloc split: fresh allocations use
malloc (so untouched tails stay lazy) and only explicit do_calloc
requests (new_array0) are zeroed.  Callers that need zeroed memory
already ask for it, and the full test suite passes.

Thanks to @guilherme-puida for the report (#959).

Fixes: #959
(cherry picked from commit 4bfd18d195)

util1.c

@@ -1788,8 +1788,6 @@ void *expand_item_list(item_list *lp, size_t item_size, const char *desc, int in
 				new_ptr == lp->items ? " not" : "");
 		}
 
-		memset((char *)new_ptr + lp->malloced * item_size, 0,
-		       (expand_size - lp->malloced) * item_size);
 		lp->items = new_ptr;
 		lp->malloced = expand_size;
 	}

util2.c

@@ -79,7 +79,9 @@ void *my_alloc(void *ptr, size_t num, size_t size, const char *file, int line)
 			who_am_i(), do_big_num(max_alloc, 0, NULL), src_file(file), line);
 		exit_cleanup(RERR_MALLOC);
 	}
-	if (!ptr || ptr == do_calloc)
+	if (!ptr)
+		ptr = malloc(num * size);
+	else if (ptr == do_calloc)
 		ptr = calloc(num, size);
 	else
 		ptr = realloc(ptr, num * size);