printf decimal integer formatting: shave off one division

once the remaining value is less than 10, the modulo operation to
produce the final digit and division to prepare for next loop
iteration can be dropped. this may be a meaningful performance
distinction when formatting low-magnitude numbers in bulk, and should
never hurt.

based on patch by Viktor Reznov.

src/stdio/vfprintf.c

@@ -166,7 +166,8 @@ static char *fmt_u(uintmax_t x, char *s)
 {
 	unsigned long y;
 	for (   ; x>ULONG_MAX; x/=10) *--s = '0' + x%10;
-	for (y=x;           y; y/=10) *--s = '0' + y%10;
+	for (y=x;       y>=10; y/=10) *--s = '0' + y%10;
+	if (y) *--s = '0' + y;
 	return s;
 }