MINOR: h2: update the {MUX,DEM}_{M,D}ALLOC flags on buffer availability

MINOR: h2: update the {MUX,DEM}_{M,D}ALLOC flags on buffer availability

Flag H2_CF_DEM_DALLOC is set when the demux buffer fails to be allocated
in the recv() callback, and is cleared when it succeeds.

Both flags H2_CF_MUX_MALLOC and H2_CF_DEM_MROOM are cleared when the mux
buffer allocation succeeds.

In both cases it will be up to the callers to report allocation failures.