(pgg-gpg-select-matching-key): Fixed: look at the right part of the

(pgg-gpg-select-matching-key): Fixed: look at the right part of the
decoded armor to find the key-identifier.
(pgg-gpg-lookup-key-owner): New function to return the
human-readable identifier of a key owner.
(pgg-gpg-lookup-id-from-key-owner): Make it easy to identify the
key itself.
(pgg-gpg-decrypt-region): Prompt with the key owner (rather
than the key value) if we have a key and can match it against a
secret key.  Also, added an XXX note pointing out fact that the
prompt only indicates the first matching key.
(pgg-pgp-encrypt-region)
(pgg-pgp-encrypt-symmetric-region, pgg-pgp-encrypt-symmetric)
(pgg-pgp-encrypt, pgg-pgp-decrypt-region, pgg-pgp-decrypt)
(pgg-pgp-sign-region, pgg-pgp-sign): Add optional 'passphrase'
argument to all these routines, so the passphrase can be managed
externally and passed in to the system.
(pgg-gpg-possibly-cache-passphrase): Add optional
'notruncate' argument, so the passphrase cache can be used
reliably with identifiers besides a pgp packet's key id.
(pgg-gpg-encrypt-symmetric-region): New function for
symmetric encryption.
(pgg-gpg-symmetric-key-p): New function to check for an symmetric
encrypted session key.
(pgg-gpg-decrypt-region): When decrypting a symmetric encrypted
message ask for the passphrase in a proper way.