apps/rehash.c: avoid printf format warning [-Wformat]

apps/rehash.c: avoid printf format warning [-Wformat]

The `aarch64-linux-android33-clang` cross-compiler (v14.0.6)
complains twice about an unsupported '%n' format specifier,
preventing a successful `--strict-warnings` build:

    error: '%n' specifier not supported on this platform [-Werror,-Wformat]
                BIO_snprintf(buf, buflen, "%s%s%n%08x.%s%d",

This is a false positive, because BIO_snprintf() implements its
own format parsing (which is implemented in the _dopr() function).

This commit fixes the problem by rewriting the code to dispense with
the dubious '%n' format specifier. As a side-effect, the code becomes
a little bit more comprehensible and self-explaining.

Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/22511)