[MI] enabling non-stop mode

[Vladimir Prus]

Hello,

I'm thinking about the best interface to query for, and enable non-stop
mode when using MI.

We presently have the -list-features command that list various things GDB
supports.

I've already proposed to add -enable-feature command to enable things
that are off by default.

Probably, we can extend this to enable feature that are not MI proper,
say:

	(gdb) -list-features
        ^done,features=[....,"non-stop",....]
	(gdb) -enable-feature non-stop
	^done

The only issue here is that non-stop mode availability in general
depends on the target, so maybe we should have a separate commands
to list "target" feature and then enable target features?

Thoughts?

- Volodya