linuxcnc.in,pickconfig.tcl: support .demo files

Thanks to Chris Morley for suggestion

A new file type (.demo) in the configs tree is supported by pickconfig.

A .demo file must be executable, typically a shell script, that invokes an app
with appropriate parameters.

This patch makes it possible to demonstrate apps related to linuxcnc using
pickconfig without starting linuxcnc or requiring a complete ini file.

The initial apps included are:

    configs/apps/halrun.demo
    configs/apps/info.demo

    configs/apps/pyvcp/pyvcp_demo1.demo

    configs/apps/latency/latency-test.demo
    configs/apps/latency/latencyplot.demo
    configs/apps/latency/latencyhistogram.demo

    configs/apps/xhc-hb04/xhc-hb04.demo
    configs/apps/xhc-hb04/xhc-hb04-layout1.demo
    configs/apps/xhc-hb04/xhc-hb04-layout2.demo