* unset XAUTHORITY env var for the networking command
When Teleport passes almost all the environment variables to the child
processes that are born of a reexec. This is an issue when XAUTHORITY is
set as it will be passed to the networking process, causing `xauth` to
use a different file than the one that the SSH session process will use.
This is because the SSH session process only inherits a small subset of
the environment variables from the parent process, and XAUTHORITY is not
one of them. This causes X11 forwarding to fail.
* add Remove method to SafeEnv