MINOR: htx: Add 3 flags on the start-line to deal with the request schemes

MINOR: htx: Add 3 flags on the start-line to deal with the request schemes

The first one, HTX_SL_F_HAS_SCHM, will be used to know the request has an
explicit scheme. So, in H2, it is always true because the pseudo-header
":scheme" is mandatory. In H1, it is only true when an absolute URI is found on
the start-line. The other flags, HTX_SL_F_SCHM_HTTP and HTX_SL_F_SCHM_HTTPS,
will be used to know which scheme the request have. For now, other protocols are
not handled.

The aim of these flags is to pass this information to the backend side in
general, and to the H2 mux in particular. So the multiplexer will have a chance
to use this information to send the right scheme to the server.