server: add option to stay in foreground

[jkoelker]

Adds an option (-D) to prevent forking into the background. Allows running mosh-server directly as a docker ENTRYPOINT[1]. The server can then be specified to use the container like:

mosh --server='function s() { CONTAINER=$((docker inspect --format={{.Id}} mosh 2> /dev/null || docker run -dti --rm -v /var/run/docker.sock:/var/run/docker.sock --name mosh --net=host jkoelker/mosh-server new -D -@ "$@") | sed '/^$/d') ; docker logs "${CONTAINER}"; }; s' [user@]host [command...]

This is particularly useful to mosh'ing into GCP Container-Optimized where the filesystem is readonly.

Fixes #1091

[1] https://github.com/jkoelker/dockerfiles/blob/mosh-server/Dockerfile#L32