mod_logio: new format-specifier %S (sum) which is the sum of received

and sent byte counts.
PR54015 [Martijn T. <apache_bugzilla@mindcontrolled.nl>, Christophe JAILLET]

trunk: http://svn.apache.org/r1484910
       http://svn.apache.org/r1517386

Submitted by: jailletc36
Reviewed by: jim, humbedooh


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1523269 13f79535-47bb-0310-9956-ffa450edef68