Missed a spot in the net/http docs. Calling Server.Shutdown() causes
ListenAndServe to return *immediately*, and when it does main() exits and
the "Upload Success" message gets cut off before it can be fully sent, which
spooks HTTP clients a bit.
To avoid that, you have to wait for the call to Server.Shutdown() to return,
and then signal back to main() that it's now safe to exit.
