CLEANUP: mux-h2: use LIST_ADDED() instead of LIST_ISEMPTY() where relevant

CLEANUP: mux-h2: use LIST_ADDED() instead of LIST_ISEMPTY() where relevant

Lots of places were using LIST_ISEMPTY() to detect if a stream belongs
to one of the send lists or to detect if a connection was already
waiting for a buffer or attached to an idle list. Since these ones are
not list heads but list elements, let's use LIST_ADDED() instead.