event no longer requires APR_POLLSET_THREADSAFE, or any

other non-standard feature (all but OS/2 have APR_POLLSET_WAKEABLE)

config-foo: don't require thread-safe pollset in order to
            build event

event.c: don't stress APR_ENOTIMPL in apr_pollset_create errors;
         just give the standard "check system or user limits"
         message


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1203404 13f79535-47bb-0310-9956-ffa450edef68