REORG: mux-h2: extract flags and enums into mux_h2-t.h

REORG: mux-h2: extract flags and enums into mux_h2-t.h

Originally in 1.8 we wanted to have an independent mux that could possibly
be disabled and would not impose dependencies on the outside. Everything
would fit into a single C file and that was fine.

Nowadays muxes are unavoidable, and not being able to easily inspect them
from outside is sometimes a bit of a pain. In particular, the flags utility
still cannot be used to decode their flags.

As a first step towards this, this patch moves the flags and enums to
mux_h2-t.h, as well as the two state decoding inline functions. It also
dropped the H2_SS_*_BIT defines that nobody uses. The mux_h2.c file remains
the only one to include that for now.