Add sapi_header_op interface which supersedes the sapi_add_header and _ex

Add sapi_header_op interface which supersedes the sapi_add_header and _ex
calls.

Revert the change to the sapi_add_header_ex interface.

Fix various bugs:

1.  header("HTTP/1.0 306 foo");
    header("Location: absolute-uri");

    did not work in combination with several SAPI modules, because
    http_status_line was never properly reset.  And thus, all SAPI
    modules which looked at http_status_line ignored the changed
    http_response_code.

2.  The CGI SAPI did not send out the HTTP status line at all, if
    http_status_line had not been set explicitly by calling
    header("HTTP/1.0 200 foo");