Support User Environment

[russjones]

When creating a new session with OpenSSH you inherit only a limited number of environment variables:

DISPLAY
HOME
LOGNAME
MAIL
PATH
SSH_ASKPASS 
SSH_AUTH_SOCK
SSH_CONNECTION
SSH_ORIGINAL_COMMAND
SSH_TTY
TZ
USER

After that, if PermitUserEnvironment true is set in sshd_config, sshd will read in ~/.ssh/environment and set these in your environment.

At the moment, Teleport supports the first half (getting the predefined set of environment variables), Teleport should also respect and environment file if it exists and this feature is enabled on the server.

[kontsevoy]

Implementation details:

Teleport should use ~/.tsh/environment since we've made the earlier decision to NOT reuse ssh directories.

The flag to accept user environment should be supported by the CLI only (for now) for start command, i.e. this should work:

$ export FOO=BAR
$ env > ~/.tsh/environment
$ teleport start --permit-user-env=true

And if you tsh localhost (as yourself) you should see the full environment at the time of teleport start exposed to your session, so echo $FOO will print "BAR".