MINOR: mux-h1: report that a buffer allocation succeeded

MINOR: mux-h1: report that a buffer allocation succeeded

When the buffer allocation callback is notified of a buffer availability,
it will now set a MAYALLOC flag in addition to clearing the ALLOC one, for
each of the 3 levels where we may fail an allocation. The flag will be
cleared upon a successful allocation. This will soon be used to decide to
re-allocate without waiting again in the queue. For now it has no effect.

There's just a trick, we need to clear the various *_ALLOC flags before
testing h1_recv_allowed() otherwise it will return false!