BUG/MEDIUM: mux-h1: Use buf_room_for_htx_data() to detect too large messages

BUG/MEDIUM: mux-h1: Use buf_room_for_htx_data() to detect too large messages

During headers parsing, an error is returned if the message is too large and
does not fit in the input buffer. The mux h1 used the function b_full() to do
so. But to allow zero copy transfers, in h1_recv(), the input buffer is
pre-aligned and thus few bytes remains always free.

To fix the bug, as during the trailers parsing, the function
buf_room_for_htx_data() should be used instead.

This patch must be backported to 2.0 and 1.9.