Unix MPMs: Stop dropping connections when the file descriptor

Unix MPMs: Stop dropping connections when the file descriptor
is at least FD_SETSIZE.

That isn't a problem for most users (since poll() is widespread).
On old boxes where select() must be used by APR, APR needs to fail
operations that don't work with such fds.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@102419 13f79535-47bb-0310-9956-ffa450edef68