cpp: Fix adding duplicated sigs on multiple uids

* lang/cpp/src/gpgsignkeyeditinteractor.cpp: Add another
state DUPE_OK2 to allow gpg looping over all uids.

When duplicated signatures should be added to multiple
user ids GnuPG will ask for each uid so we have to
add a transition for that.

• GnuPG-Bug-Id: T4734