Rename from posix to unix to parallel x/sys/unix

Use the `_unix` file suffix to denote *NIX-like semantics in order to parallel the designation given by the package `golang.org/x/sys/unix`.

client/driver/utils_linux.go

@@ -1,16 +0,0 @@
-package driver
-
-import (
-	"os/exec"
-	"syscall"
-)
-
-// isolateCommand sets the setsid flag in exec.Cmd to true so that the process
-// becomes the process leader in a new session and doesn't receive signals that
-// are sent to the parent process.
-func isolateCommand(cmd *exec.Cmd) {
-	if cmd.SysProcAttr == nil {
-		cmd.SysProcAttr = &syscall.SysProcAttr{}
-	}
-	cmd.SysProcAttr.Setsid = true
-}