Add "AcceptErrorsNonFatal" directive

This tweaks accept() failure processing by having ap_unixd_accept
pass more errors up, and having the MPM's check against a macro
to see if they are in a whitelist of non ENETDOWN/EMFILE kind 
of potential process-wide errors.

Default behavior is still to exit.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1820098 13f79535-47bb-0310-9956-ffa450edef68