Change Lfc threading property to single-thread or non-threaded

[patilatharva]

Currently, the Lfc CLI has a --threading option which accepts a "true"/"false" value.
This is inconsistent with all of our other boolean options (e.g. --quiet, --clean), which don't require explicit arguments because they're false by default and become true when supplied.

The --threading option seems to be designed this way because unlike the other boolean options, we want it to be true by default:

lingua-franca/org.lflang/src/org/lflang/generator/c/CGenerator.java

Lines 406 to 409 in 0e00a81

	// If the unthreaded runtime is not requested by the user, use the threaded runtime instead
	// because it is the only one currently capable of handling asynchronous events.
	if (!targetConfig.threading && !targetConfig.setByUser.contains(TargetProperty.THREADING)) {
	targetConfig.threading = true;

I think it might be better if we changed it to something like --single-thread so it can be false by default, and users won't have to explicitly supply an argument for an otherwise boolean option. What are your thoughts?

--threading=false → --single-thread or --non-threaded

[edwardalee]

This is a good idea. Maybe '--no-threads'.

[lhstrh]

I also like --no-threads.

[lhstrh]

Alternatively, we could have --single-threaded. It is longer, but more technically correct...

[cmnrd]

This one is already implemented, right? I think we should consider removing the old property for the 0.7.0 milestone and open a new issue to track this.

[cmnrd]

This was implemented in #1817