gpg: Add support for unwrapping the outer level of encryption.
* g10/decrypt-data.c (decrypt_data): If OPT.UNWRAP_ENCRYPTION is set, copy the data to the output file instead of continuing to process it. * g10/gpg.c (enum cmd_and_opt_values): Add new value oUnwrap. (opts): Handle oUnwrap. (main): Likewise. * g10/options.h (opt): Add field unwrap_encryption. * g10/plaintext.c (handle_plaintext): Break the output file selection functionality into ... (get_output_file): ... this new function.