http: client keep-alive for UNIX domain sockets

[bengl]

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.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

http, test