gpg: Extend the ERRSIG status line with a fingerprint.

* g10/mainproc.c (issuer_fpr_raw): New.
(issuer_fpr_string): Re-implement using issuer_fpr_rtaw.
(check_sig_and_print): Don't free ISSUER_FPR.  Use ISSUER_FPR_RAW.
Use write_status_printf.  Extend ERRSIG status.

Modern OpenPGP implementations put the ISSUER_FPR into the signature
to make it easier to discover the, public needed to check the
signature. This is also useful in error messages and thus we add it.

• Signed-off-by: Werner Koch <wk@gnupg.org>