Update docs: `gcutil` -> `gcloud` to match code.
Remove the read-only setting on vars.

This allows us to source the settings of `$PROJECT` and `$ZONE` into other
scripts and use them as defaults with optional overrides rather than as global
constants.
Added a script and howto for using a SOCKS proxy.

Using a SOCKS proxy is much easier than individual SSH tunnels for (host, port)
pairs, especially when accessing multiple cloud VM instances, and/or when they
link to each other, as this makes it a simple, seamless user experience.