Don't reset the logger in a vfork

9c766a40cb broke logging from the
daemon, because commonChildInit is called when starting the build hook
in a vfork, so it ends up resetting the parent's logger. So don't
vfork.

It might be best to get rid of vfork altogether, but that may cause
problems, e.g. when we call an external program like git from the
evaluator.

src/libutil/util.hh

@@ -262,7 +262,7 @@ struct ProcessOptions
     string errorPrefix = "";
     bool dieWithParent = true;
     bool runExitHandlers = false;
-    bool allowVfork = true;
+    bool allowVfork = false;
 };
 
 pid_t startProcess(std::function<void()> fun, const ProcessOptions & options = ProcessOptions());