I.e. allow block-level or global modification of which user sudo executes as.
Right now, users who always need to run sudo as another user (e.g. to run commands "as" a user which does not have SSH login privileges, and which is not root, such as daemon user accounts) have no useful recourse.
Easiest solution is just a new env var that acts as a default if the kwarg is not given.
Add env.sudo_user; fixes #615