Use BSD instead of POSIX file locks

POSIX file locks are essentially incompatible with multithreading. BSD
locks have much saner semantics. We need this now that there can be
multiple concurrent LocalStore::buildPaths() invocations.

src/libstore/local-store.hh

@@ -263,7 +263,7 @@ private:
     bool isActiveTempFile(const GCState & state,
         const Path & path, const string & suffix);
 
-    int openGCLock(LockType lockType);
+    AutoCloseFD openGCLock(LockType lockType);
 
     void findRoots(const Path & path, unsigned char type, Roots & roots);