Allow signing using an exactly specified subkey.

* src/engine-gpg.c (append_args_from_signers): Detect exactly
specified keys and apped the '!' suffix.

Due to the ABI break which removed long long deprecated functions we
can also risk to introduce a slight semantic change in the way signer
keys are specified. The change is that iff a subkey-fingerprint with
the '!' suffix was used to lookup a signer's key we now use this
specific subkey and not any key gpg considers to be a good signing
subkey. Most people would have considered the old behaviour anyway as
a bug because it differs from what gpg uses at the command line.

• GnuPG-bug-id: T3325
• Suggested-by: Benjamin Kibbey