Overhaul ab.c stats collection and reporting to avoid integer

truncation and time divisions within the test loop, retain
native time resolution until output, remove unused data,
avoid structure copies, consistently round milliseconds, and
generally avoid losing accuracy of calculation due to type casts.

Incidentally fixes output bug on gnuplot (seconds were being
output as microseconds).  It would make more sense to output
gnuplot stats as microseconds after the start of test, but
this change returns us to consistency with pre-apr versions of ab.

PR: 44878, 44931.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@655214 13f79535-47bb-0310-9956-ffa450edef68