MEDIUM: htx: Rework conversion from a buffer to an htx structure

MEDIUM: htx: Rework conversion from a buffer to an htx structure

Now, the function htx_from_buf() will set the buffer's length to its size
automatically. In return, the caller should call htx_to_buf() at the end to be
sure to leave the buffer hosting the HTX message in the right state. When the
caller can use the function htxbuf() to get the HTX message without any update
on the underlying buffer.