Kill builds when we get EOF on the log FD

This closes a long-time bug that allowed builds to hang Nix
indefinitely (regardless of timeouts) simply by doing

  exec > /dev/null 2>&1; while true; do true; done

Now, on EOF, we just send SIGKILL to the child to make sure it's
really gone.

src/libmain/shared.cc

@@ -333,7 +333,7 @@ RunPager::~RunPager()
         if (pid != -1) {
             std::cout.flush();
             close(STDOUT_FILENO);
-            pid.wait(true);
+            pid.wait();
         }
     } catch (...) {
         ignoreException();