gpg: New option --rfc4880bis.

* g10/options.h (struct opt): Add field flags.rfc4880bis.
* g10/gpg.c (oRFC4880bis): new.
(opts): add --rfc4880bis.
(main): Implement that and print a warning.

This is option enables experimental features which may be dropped or
changed with the next release.

• Signed-off-by: Werner Koch <wk@gnupg.org>