ab: repeat apr_pollset_poll() while APR_STATUS_IS_EINTR(rv)

Backports: r750094
Submitted by: trawick
Reviewed by: sf, wrowe


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1005524 13f79535-47bb-0310-9956-ffa450edef68