gpg: Allow import of large keys.

* g10/import.c (import): Skip too large keys.
* kbx/keybox-file.c (IMAGELEN_LIMIT): Change limit from 2MB to 5MB.

The key which triggered the problem was 0x57930DAB0B86B067. With this
patch it can be imported. Keys larger than the now increased limit of
5MB will are skipped and the already existing not_imported counter is
bumped up.

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