Fix halcmd debug (uspace only)

[hdiethelm]

As described in #3866 the hallcmd debug was not working.

I fixed this by forwarding the command to rtapi_app and add the debug command also in there.

Caviat: This works only in uspace mode.

ToDo: Test with rtlinux. I tested only without so far.

[hdiethelm]

My testing version with some additional printf's is here if someone wants to see how this works:
https://github.com/hdiethelm/linuxcnc-fork/tree/fix_debug_hacking

If I find time, i will test it also on rtlinux. If someone else has time, feel free to test it also.

Sadly, I can not correct this for RTAI. I do not have a setup to test it. But I added an error message to notify others that this is not yet implemented.

[hdiethelm]

How I tested (from src folder):

• Open LinuxCNC, axis_mm example: ../scripts/linuxcnc
• G0 X10 Y0
• No output
• ../bin/halcmd debug 5
• G0 X0 Y0
• Terminal shows: SET_LINE
• ../bin/halcmd debug 1
• G0 X10 Y0
• No output

So far I tested in a VM.
RTLinux and Xenomai work fine.