msg-sim: expose simple API for writing sysctl networking options #165

Context

Every namespace in msg-sim has its own virtual proc filesystem with various networking-related kernel parameters. This fs is mounted in every namespace, so it can be changed by writing to the correct file paths (#164).

I think it's worth it to expose the relevant setters & getters as either helper functions, or methods on Ctx for the following reasons:

• Make them explicitly discoverable
• Better input validation and error handling

We could even have a custom build script that probes sysctl options and kernel version etc and generates helpers and validation based on this, but that's a nice to have.