* src/gpgme.h.in (GPGME_DECRYPT_ARCHIVE): New decryption flag.
* src/engine-gpg.c (gpg_decrypt): Set use_gpgtar engine flag if
GPGME_DECRYPT_ARCHIVE flag is set. Check for new enough gpg and
incompatible flags. Use add_gpg_arg_with_value for gpg-only options
with a value and add_gpg_arg for gpg-only options without a value.
Set extra options for gpgtar and pass input data to stdin when using
gpgtar.
* tests/run-decrypt.c (show_usage): New options --archive and
--directory.
(main): Parse new options. Decrypt with gpgtar if --archive is given.
Set file name of output data to value of --directory option.