json: Add proper decrypt_result_t handling

* src/gpgme-json.c (recipient_to_json, decrypt_result_to_json):
New.
(op_decrypt, hlp_decrypt): Update.

The op_decrypt as one of the first operations did not yet
match the current 1 <> 1 mapping of gpgme types to json
dictonaries.

info and dec_info are bad names but used for compatibility reasons.