MINOR: mux-h1: Set flags about the request's scheme on the start-line

MINOR: mux-h1: Set flags about the request's scheme on the start-line

We first try to figure out if the URI of the start-line is absolute or not. So,
if it does not start by a slash ("/"), it means the URI is an absolute one and
the flag HTX_SL_F_HAS_SCHM is set. Then checks are performed to know if the
scheme is "http" or "https" and the corresponding flag is set,
HTX_SL_F_SCHM_HTTP or HTX_SL_F_SCHM_HTTPS. Other schemes, for instance ftp, are
ignored.