Fixed Windows build by guarding Unix socket code.
Fixed Windows build by guarding Unix socket code.

We kept the `csi/paths.cpp` file in the build on Windows because
`slave/paths.cpp` now depends on it, and the only "broken" part is the
use of Unix sockets in `getEndpointSocketPath()`. For now, we will
return an error instead of trying to use unsupported Unix sockets on
Windows.

kaysoky/mesos,andschwa/mesos,dforsyth/mesos,kaysoky/mesos,chhsia0/mesos,gsantovena/mesos,chhsia0/mesos,jpeach/mesos,jpeach/mesos,Gilbert88/mesos,reneploetz/mesos,asamerh4/mesos,jpeach/mesos,kaysoky/mesos,dforsyth/mesos,jpeach/mesos,abudnik/mesos,chhsia0/mesos,gsantovena/mesos,andschwa/mesos,chhsia0/mesos,gsantovena/mesos,Gilbert88/mesos,gsantovena/mesos,Gilbert88/mesos,abudnik/mesos,dforsyth/mesos,andschwa/mesos,asamerh4/mesos,asamerh4/mesos,gsantovena/mesos,kaysoky/mesos,andschwa/mesos,reneploetz/mesos,reneploetz/mesos,chhsia0/mesos,jpeach/mesos,dforsyth/mesos,dforsyth/mesos,asamerh4/mesos,asamerh4/mesos,jpeach/mesos,Gilbert88/mesos,abudnik/mesos,dforsyth/mesos,reneploetz/mesos,kaysoky/mesos,asamerh4/mesos,abudnik/mesos,abudnik/mesos,chhsia0/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,jpeach/mesos,asamerh4/mesos,andschwa/mesos,abudnik/mesos,dforsyth/mesos,reneploetz/mesos,andschwa/mesos,andschwa/mesos,Gilbert88/mesos,Gilbert88/mesos,kaysoky/mesos,gsantovena/mesos,gsantovena/mesos,reneploetz/mesos,reneploetz/mesos,chhsia0/mesos