unifdef: Handle redefined symbols correctly.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D41758

unifdef: Reapply our 61287be18189.

Add 'contrib/unifdef/' from commit '0da44885831dc0a43c4ca6ff04a2430993cc0a80'

git-subtree-dir: contrib/unifdef
git-subtree-mainline: 3b7ffacdee49f90716cba2bcf8af3fc1971ae031
git-subtree-split: 0da44

Add 'contrib/unifdef/' from commit '0da44885831dc0a43c4ca6ff04a2430993cc0a80'

git-subtree-dir: contrib/unifdef
git-subtree-mainline: 3b7ffacdee49f90716cba2bcf8af3fc1971ae031
git-subtree-split: 0da44885831dc0a43c4ca6ff04a2430993cc0a80

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