gpg: Fix using --decrypt along with --use-embedded-filename.

* g10/options.h (opt): Add flags.dummy_outfile.
* g10/decrypt.c (decrypt_message): Set this global flag instead of the
fucntion local flag.
* g10/plaintext.c (get_output_file): Ignore opt.output if that was
used as a dummy option aslong with --use-embedded-filename.

The problem here was that an explicit specified --decrypt, as
meanwhile suggested, did not work with that dangerous
--use-embedded-filename. In contrast it worked when gpg decrypted as
a side-effect of parsing the data.

• GnuPG-bug-id: T4500
• Signed-off-by: Werner Koch <wk@gnupg.org>