Summary:
Ref T13395. Merge a lot of stuff which doesn't break existing workflows:

    - Merge a UTF8 fix for "cmd.exe" on Windows.
    - Merge minor changes to JSON linters.
    - Merge some shell completion stuff.
    - Merge some "arc anoid" fixes.
    - Merge various Windows improvements to unit tests which interact with processes / the filesystem.
    - Merge some other Windows path fixes.
    - Merge a UTF8 character class fix.
    - Merge script initialization.
    - Merge unit test support scripts.
    - Merge some initialization code.
    - Merge Windows stdout/stderr-as-files code.
    - Merge a bunch of code for making exec tests work on Windows.
    - Merge more Windows unit test fixes.
    - Merge "continue on failure" mode when loading symbols.
    - Merge "GPC" order CLI fixes.

Test Plan: Ran `arc unit --everything`; created this change. There's likely some less-than-perfect code here.

Maniphest Tasks: T13395

Differential Revision: https://secure.phabricator.com/D20988