Introduce whine_realloc

Move few patters with whine_malloc, if (successful) copy+free, to a new
whine_realloc. It should do the same thing, but with a help from OS it
can avoid unnecessary copy and free if allocation of more data after
current data is possible.

Added few setting remanining space to 0, because realloc does not use
calloc like whine_malloc does. There is no advantage of zeroing what we
will immediately overwrite. Zero only remaining space.

src/cache.c

@@ -1676,10 +1676,8 @@ int cache_make_stat(struct txt_record *t)
 	      {
 		/* expand buffer if necessary */
 		newlen = bytes_needed + 1 + bufflen - bytes_avail;
-		if (!(new = whine_malloc(newlen)))
+		if (!(new = whine_realloc(buff, newlen)))
 		  return 0;
-		memcpy(new, buff, bufflen);
-		free(buff);
 		p = new + (p - buff);
 		lenp = p - 1;
 		buff = new;