Security: dbus: investigate and mitigate command injection

Nobody has claimed this yet.

Due to the way we are passing arguments along, we should be more careful in testing for side effects when a user tries to call dfx-mgr (or other future softeners).

I see a few tasks/options:
- see how much of an issue this is
- reimplement the dfx-mgr argument parser and require us to maintain it (burden) alongside dfx-mgr upstream
- add fuzz testing
- add side effect tests (try to get around the protections and create files or whatever other harmless, easily detected side effects can be produced and then test against them in future) whackamole style
- rely on an external crate or otherwise get experienced eyes on this

Start by tracing how fpgad passes arguments to dfx-mgr and review the existing protections around that call; the issue names no files or tests. Reproduce harmless side effects to establish the current risk, then define completion as a mitigation with regression, fuzz, or side-effect test coverage.

Written by the indexing model from the issue text.