http: client keep-alive for UNIX domain sockets

Makes `Connection: keep-alive` behave correctly when making client
connections to UNIX domain sockets.

Prior to this, connections would never be re-used, but the keep-alive
would cause the connections to stick around until they time out. This
would lead to an eventual EMFILE error due to all the connections
staying open. This was due to http.Agent not properly supporting UNIX
domain sockets.

PR-URL: https://github.com/nodejs/node/pull/13214


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>