• src/engine-gpg.c (engine_gpg): Add diagnostics member.

(gpg_release): Release diagnostics data.
(gpg_new): Set up logger-fd and diagnostics.
(gpg_getauditlog): New. Copy diagnostics to a user data.
(engine_ops): Add getauditlog.

• src/getauditlog.c (getauditlog_start): Don't reset engine

for diagnostics.

• src/gpgme.h.in (GPGME_AUDITLOG_DIAG): New.
• tests/run-decrypt.c (show_usage, main): Add --diagnostics.

This enables users of GPGME to get more verbose information
from gpg which can assist users in figuring out a problem
that was before hidden behind a generalized error like
"Decryption Failed".

Tested with run-decrypt