ab: Fix maintenance of the pollset to resolve EINPROGRESS errors

with kqueue (BSD/OS X) and excessive CPU with event ports (Solaris).

The apr_pollset API requires that a socket be removed from
the pollset whenever there is a change to the requested events.
The socket must also be removed when it is closed.  This wasn't
completely implemented.

PR: 44584


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@749438 13f79535-47bb-0310-9956-ffa450edef68