MINOR: htx: Be sure to xfer all headers in one time in htx_xfer_blks()

MINOR: htx: Be sure to xfer all headers in one time in htx_xfer_blks()

In the function htx_xfer_blks(), we take care to transfer all headers in one
time. When the current block is a start-line, we check if there is enough space
to transfer all headers too. If not, and if the destination is empty, a parsing
error is reported on the source.

The H2 multiplexer is the only one to use this function. When a parsing error is
reported during the transfer, the flag CS_FL_EOI is also set on the conn_stream.