name protect unixd_setup_child and restore its prototype to unixd.h

If someone wants to find a better place for it, then feel free to
so so only if you compile with -Werror before committing.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@739151 13f79535-47bb-0310-9956-ffa450edef68