Rename `Cmd.Exec` to `Cmd.Spawn`

This is to `spawn` a process not to `exec` one
Use `Exec` except Windows for the last command run in the commands chain

This is to avoid the interrupt/pagination issues like https://github.com/github/hub/issues/708. Note that `exec(3)` is not available on Windows: http://golang.org/src/pkg/syscall/exec_windows.go#L339. A fall to `spawn(3)` is used in this case.
Rename `Cmd.ExecOutput` to `Cmd.CombinedOutput` to avoid confusion

We now have `Cmd.Spawn` and `Cmd.Exec`, to avoid confusion, we use `Cmd.CombinedOutput` for combined output.
Merge pull request #714 from github/exec_cmd

Use `Exec` except Windows for the last command run in the commands chain