Conditional kill

Currently, sclang will be killed when any vim session containing sc code closes.

With this patch, only closing the vim session that opened sclang will kill it. Closing any other vim session with sc code will have no effect.

This doesn't matter if using vim properly with buffers.

But if you use a different vim for each file in different terminals you wonder a lot why your sclang got killed.

The patch should have no effect when using only one vim with buffers.

Let me know if you want to pull but can't because you're not happy with coding style or other requirements, I'll happily fix it.