Fixes #418 and #931. Unix terminal hangs after exit.

[BDisp]

Also fixes #418.

[BDisp]

Is preferable stay with this issue than merge this PR until the screen resize is fixed too.

[migueldeicaza]

Ok, I read the issue mentioned here, and this seems to be a disagreement between .NET Core and PowerShell and Gui.cs, because whatever code is in PowerShell is incomplete, and should be using the proper Application Mode sequences.

That said, I can understand why this would be desirable, and I think that rather than adding a custom hack (and one that should really use tigetstr ("smkx") and tigetstr ("rmkx")) that we should introduce two API entry points: Enter/Leave keyboard-application mode, and the PowerShell integration would have to explicitly enable/disable those, rather than hiding these inside the regular codepath - and also hardcoding these constatnts.

[tig]

Wow! Is this really all that's needed to fix this?

Since I could never repro the issue, I'm nervous about declaring victory. I'll merge this and then we can ask folks to verify.