Add a --repair flag to ‘nix-store -r’ to repair derivation outputs

With this flag, if any valid derivation output is missing or corrupt,
it will be recreated by using a substitute if available, or by
rebuilding the derivation.  The latter may use hash rewriting if
chroots are not available.

src/libstore/local-store.hh

@@ -150,7 +150,7 @@ public:
 
     Paths importPaths(bool requireSignature, Source & source);
 
-    void buildPaths(const PathSet & paths);
+    void buildPaths(const PathSet & paths, bool repair = false);
 
     void ensurePath(const Path & path);
 
@@ -202,6 +202,10 @@ public:
        a substituter (if available). */
     void repairPath(const Path & path);
 
+    /* Check whether the given valid path exists and has the right
+       contents. */
+    bool pathContentsGood(const Path & path);
+
 private:
 
     Path schemaPath;