MINOR: htx: Add functions to get the first block of an HTX message

MINOR: htx: Add functions to get the first block of an HTX message

It is the first block relatively to the start-line. So it is the start-line if
its position is set (sl_pos != -1), otherwise it is the head. The functions
htx_get_first() and htx_get_first_blk() can be used to get it.  This change is
mandatory to consider 1xx informational messages as part of a response.