mongocryptd uses a PID file to act as a singleton process. By default it creates the PID file in the current working directory. Instead, we should use a sensible user-specific default path, determined by libmongocrypt in CDRIVER-3260.

Similarly, we should choose a user specific domain socket path.

Also consider having drivers read the PID file. This could detect if a user already has a spawned mongocryptd listening on a different port than what they're attempting to spawn/connect to.