Default to 1 interactive thread

[IanButterworth]

Part of #43672

So:

• default == 1 default, 1 interactive
• -t1 == 1 default, 1 interactive
• -t1,0 == 1 default, 0 interactive
  etc.

Motivations

• Moves the IO loop off the main thread
• Makes the default case allow the REPL to do threaded things in the background i.e. completion hints that could be cancelled on next keystroke

[IanButterworth]

Suggested change

	// By default, main threads = -1 (== "auto"), interactive = 1
	long nthreads = -1, nthreadsi = 1;
	// By default:
	// default threads = -1 (== "auto")
	long nthreads = -1;
	// interactive threads = 1, or 0 if generating output
	long nthreadsi jl_generating_output() ? 0 : 1;