add ap_get_status_line() so that modules can get a standardized

    Status-Line value for their response.

Submitted by: Joe Orton <joe@orton.demon.co.uk>
Reviewed by: Greg Stein


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85762 13f79535-47bb-0310-9956-ffa450edef68