MINOR: mux-h2: add a new function h2s_alert() to call the data layer

MINOR: mux-h2: add a new function h2s_alert() to call the data layer

In order to report an error to the data layer, we have different ways
depending on the situation. At a lot of places it's open-coded and not
always correct. Let's create a new function h2s_alert() to handle this
task. It tries to wake on recv() first, then on send(), then using
wake().