Make --repair-path also repair corrupt optimised links

There already existed a smoke test for the link content length,
but it appears that there exists some corruptions pernicious enough
to replace the file content with zeros, and keeping the same length.

--repair-path now goes as far as checking the content of the link,
making it true to its name and actually repairing the path for such
coruption cases.

src/libutil/logging.hh

@@ -40,7 +40,7 @@ struct LoggerSettings : Config
     Setting<bool> showTrace{
         this, false, "show-trace",
         R"(
-          Where Nix should print out a stack trace in case of Nix
+          Whether Nix should print out a stack trace in case of Nix
           expression evaluation errors.
         )"};
 };