[LinuxCNC/linuxcnc PR#553] Zultron/lcnc pr docker cleanup fixes

Here are a few cleanups to the scripts/linuxcnc.in template.

The most significant update is ignoring defunct zombie processes when cleaning up on exit. These hang around in Docker containers, and aren’t a problem except where they trigger false positives in scripts like this. More info [here][1].

The others are minor: Use the autoconfigured grep executable, and fix a log message.

[1]: https://blog.phusion.nl/2015/01/20/docker-and-the-pid-1-zombie-reaping-problem/