For the pkg/expect package, if the process has been stopped but there is
no `Close()` call, the `ExitCode()` won't return exit code correctly.
The `ExitCode()` should check `exitErr` and return exit code if cmd isn't nil.
And introduces `exitCode` to return correct exit code based on the
process is signaled or exited.
Signed-off-by: Wei Fu <fuweid89@gmail.com>
b3316c0e09