A corrupt entry in .links prevents adding a fixed version of that file
to the store in any path.  The user experience is that corruption
present in the store 'spreads' to new paths added to the store:

(With store optimisation enabled)

1. A file in the store gets corrupted somehow (eg: filesystem bug).
2. The user tries to add a thing to the store which contains a good copy
   of the corrupted file.
3. The file being added to the store is hashed, found to match the bad
   .links entry, and is replaced by a link to the bad .links entry.
   (The .links entry's hash is not verified during add -- this would
   impose a substantial performance burden.)
4. The user observes that the thing in the store that is supposed to be
   a copy of what they were trying to add is not a correct copy -- some
   files have different contents!  Running "nix-store --verify
   --check-contents --repair" does not fix the problem.

This change makes "nix-store --verify --check-contents --repair" fix
this problem.  Bad .links entries are simply removed, allowing future
attempts to insert a good copy of the file to succeed.