MINOR: htx: Replace the function http_find_stline() by http_get_stline()

MINOR: htx: Replace the function http_find_stline() by http_get_stline()

Now, we only return the start-line. If not found, NULL is returned. No lookup is
performed and the HTX message is no more updated. It is now the caller
responsibility to update the position of the start-line to the right value. So
when it is not found, i.e sl_pos is set to -1, it means the last start-line has
been already processed and the next one has not been inserted yet.

It is mandatory to rely on this kind of warranty to store 1xx informational
responses and final reponse in the same HTX message.