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

CC: aran

Maniphest Tasks: T550

Differential Revision: https://secure.phabricator.com/D4232