Mirror env() values back to the process

[wduquette]

In standard TCL, changes to the env() array are mirrored back to the process, and will affect any subprocesses spawned by the Molt process. Given that Molt currently does not support the standard TCL exec and open commands, this is not a crucial problem; but it ought to be fixed eventually.

[wduquette]

Need to check standard TCL for full semantics, e.g.,

• What happens if the env() variable is unset?
• Does the mirroring use variable tracing, or is it some other mechanism.