BUG/MEDIUM: mux-h1: make HTX chunking consistent with H2

BUG/MEDIUM: mux-h1: make HTX chunking consistent with H2

When transfering from H1 to H1, chunking is always indicated by the
presence of the Transfer-encoding header field. But when a message
comes from H2 there is no such header and only HTX_SL_F_XFER_LEN
ought to be relied on. This one will also result in H1_MF_XFER_LEN
to be set, just like transfer-encoding, so let's always rely on
this latter flag to detect the need for chunking (when CLEN is not
here) and automatically add the transfer-encoding header if it was
not present, as reported by H1_MF_CHNK.

This must be backported to 1.9.