Merge r1592529 from trunk:

mod_proxy_scgi: Support Unix sockets.

ap_proxy_port_of_scheme(): Support default SCGI port (4000).

Submitted by: trawick
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1593004 13f79535-47bb-0310-9956-ffa450edef68