Avoid interleaving stderr in dump_fingerprint

Avoid interleaving stderr in dump_fingerprint

* src/fns.c (hexbuf_digest): New function, containing most of
the old make_digest_string.
(make_digest_string): Use it.
* src/pdumper.c (dump_fingerprint): Rewrite to use a single
fprintf call, to avoid interleaving on GNU/Linux.