R1:6dd01698732f

Fix various issues with SSH receivers

Summary:
- Original command is in SSH_ORIGINAL_COMMAND, not normal argv.
- Use PhutilShellLexer to parse it.
- Fix a protocol encoding issue with ConduitSSHWorkflow. I think I'm going to make this protocol accept multiple commands anyway because SSH pipes are crazy expensive to build (even locally, they're ~300ms).

Test Plan: With other changes, successfully executed "arc list --conduit-uri=ssh://localhost:2222".

Reviewers: btrahan, vrana

Reviewed By: btrahan…