Add a mosh module

[lloeki]

mosh is quite a useful service that people like to have auto-started.

Having it descriptively managed OOTB would be nice.

There's a weird sandboxing issue to know about though: mobile-shell/mosh#1254

[emilazy]

FWIW, I was a happy Mosh user for many years but it’s in quite a bad maintenance state for a security-critical project – no release in over two years, no commits for over a year, and years‐long periods of inactive development. I think the original authors have moved on and sadly I’m not sure it’s in a state where we should be encouraging people to use it. Something like the (suboptimally‐named) SSH3 may be a good option for gaining a subset of its benefits on top of a better‐maintained memory‐safe codebase with more standard cryptographic protocols. Unfortunately I don’t think there is anything that replicates the terminal state synchronization model that makes Mosh good at handling even the very worst of connections.

All that said, we might still merge a module if someone contributed one.