agent: Default to extended key format.

* agent/gpg-agent.c (oDisableExtendedKeyFormat, oNoop): New.
(oEnableExtendedKeyFormat): Remove.
(opts): Make --enable-extended-key-format a dummy option.  Add
disable-extended-key-format.
(parse_rereadable_options): Implement oDisableExtendedKeyFormat.

Extended key format is supported since vesion 2.1.12 which should have
long been replaced by a newer version inh all installations. Thus for
2.3 we will make use of the extended-key-format by default.

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