Add the ability to benchmark POST requests.

Note I tweaked Kurt's submission to fit our style guide... and I corrected
a few comments and crud that have been in ab.c since the zb.c days.  And
for some reason ab was printing the results twice for me, I fixed that.

PR:             2871
Submitted by:   Kurt Sussman <kls@best.com>  [modified by Dean]


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@81946 13f79535-47bb-0310-9956-ffa450edef68