* pgg-gpg.el (pgg-gpg-possibly-cache-passphrase): New optional

* pgg-gpg.el (pgg-gpg-possibly-cache-passphrase): New optional
parameter key, overrides the key id used to store passphrase
under (uses true key id from gpg output if nil).
(pgg-gpg-encrypt-region): Search for passphrase using user suplied
string STR, instead of (pgg-lookup-key STR t).
(pgg-gpg-encrypt-region): Store passphrase under user suplied
string, instead of real key id taken from gpg output.
(pgg-gpg-decrypt-region): Likewise.
(pgg-gpg-sign-region): Likewise.
* pgg.el (pgg-decrypt-region): Don't set pgg-default-user-id.