unifdef: Vendorize.
(cherry picked from commit 7addfafe73e077891f003b7cc7a62cfb434f92ec)
unifdef: Handle redefined symbols correctly.
MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D41758
(cherry picked from commit aacbe7384221d2eafa326864bbbe2f22a10063ce)