Also differentiate default from system shell

Fixes #75509

Part of #75091

Fixes #70248

Fixes #73662

Fixes Microsoft/vscode-remote-release#38

Part of #69115

* adopt paths.node over path

* fix strict null

* remove hacky typing inlabel.test.ts

Part of #64897

Part of #63832

* Manually added strict null checks in around 70 files

* Fixed tsconfig.strictNullChecks.json merge fail

* Touched up a few unnecessary type changes

* Removed files already in other PRs

- False now really means false
- The default is true for all platforms

Fixes #65037

Part of #65035

This reverts commit dbdbbe52.

Part of #60565

Fixes #58015