fix printf regression with alt-form octal, zero flag, and field width

commit b91cdbe2bc8b626aa04dc6e3e84345accf34e4b1, in fixing another
issue, changed the logic for how alt-form octal adds the leading zero
to adjust the precision rather than using a prefix character. this
wrongly suppressed the zero flag by mimicing an explicit precision
given by the format string. switch back to using a prefix character.

based on bug report and patch by Dmitry V. Levin, but simplified.

src/stdio/vfprintf.c

@@ -570,7 +570,7 @@ static int printf_core(FILE *f, const char *fmt, va_list *ap, union arg *nl_arg,
 			if (0) {
 		case 'o':
 			a = fmt_o(arg.i, z);
-			if ((fl&ALT_FORM) && p<z-a+1) p=z-a+1;
+			if ((fl&ALT_FORM) && p<z-a+1) prefix+=5, pl=1;
 			} if (0) {
 		case 'd': case 'i':
 			pl=1;