printf: allow 0 as a flag and allow multiple flags

The '%' character in a format specification may be followed by
one or more flags from the list "+- #0".  BusyBox printf didn't
support the '0' flag or allow multiple flags to be provided.
As a result the formats '%0*d' and '%0 d' were considered to be
invalid.

The lack of support for '0' was pointed out by Andrew Snyder on the
musl mailing list:

   https://www.openwall.com/lists/musl/2021/12/14/2

function                                             old     new   delta
printf_main                                          860     891     +31
.rodata                                            99281   99282      +1
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 32/0)               Total: 32 bytes

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

coreutils/printf.c

@@ -313,7 +313,7 @@ static char **print_formatted(char *f, char **argv, int *conv_err)
 				}
 				break;
 			}
-			if (*f && strchr("-+ #", *f)) {
+			while (*f && strchr("-+ #0", *f)) {
 				++f;
 				++direc_length;
 			}