Consider security of environment

[jupyterhub]

Consider security of environment

[rkdarst]

Let's face it, right now almost every spawner evaluates shell startup files for users. SlurmSpawner did, even with --keepvars, because srun was not used which was required for this to take effect. Limiting environment then broke users' software that was being run. I don't have time to go through it all now, but let's just say that properly fixing things will require a detailed, spawner-specific analysis and will likely impact some users.

#97 at least now mentions that this is a currently unsolved problem and we can't guarantee security.

[rkdarst]

Also if environment variables are used to authenticate as an admin, they are usually going to be passed to the user as well. req_keepvars can stop that but it's unwieldy to use. Consider this, too.