gpg: New import option "force-update"

* g10/options.h (IMPORT_FORCE_UPDATE): New.
* g10/import.c (parse_import_options): Add "force_update".
(import_one_real): Use force_update.

Although the new database schema v2 handles ADSK and non-unique
subkeys correctly, the fingerprints of all those keys are
not automatically updated and can thus not been found using the
fingerprint. This can however easily be fixed by exporting all keys
and do an import of these keys using --import-options=force-update.

• GnuPG-bug-id: T7892