Synchronized FFI access to POSIX environment variable functions

[mathstuf]

@matklad

If we follow the path of deprecating set_env , we should provide env-less way to tweak such API, like (strawman) std::panic::enable_backtrace(yes: bool) .

IMO, (proper) APIs are always better than ambient global state manipulation . In this case, that argument really should be a #[non_exhaustive] enum of potential states, not just a bool.