remove special handling of ipython_kernel in launcher

- set JPY_PARENT_PID and JPY_INTERRUPT_EVENT env variables for all kernels
- no custom kernel args other that console passthrough
- use devnull instead of `--no-stdout` (comments claim it shouldn't work, but tests suggest it does, so let's keep an eye on this)

closes #7085
Yes, [Calico is](https://bitbucket.org/ipre/calico/src/aaaa1764c6cee067368741923e28eb5d99bce0ef/Source/Calico/Main.cs?at=master#cl-429). @minrk , I thought you'd found that and were going to set the old environment variable for backwards compatibility?