gpg: Add import option "keep-ownertrust".
* g10/options.h (IMPORT_KEEP_OWNERTTRUST): New. * g10/import.c (parse_import_options): Add "keep-ownertrust". (import_one): Act upon new option.
This option is in particular useful to convert from a pubring.gpg to
the new pubring.kbx in GnuPG 2.1 or vice versa:
gpg1 --export | gpg2 --import-options keep-ownertrust --import
(cherry-picked from commit ffc2307843ce6c4ac3c8d99ba8c70ffa1ae28e39)