apps/tunnel/tunneld: Get the listenSpec through the context.
apps/tunnel/tunneld: Get the listenSpec through the context.

This fixes the shell test.  If the program is initialized
with veyron2.Init you can't use the old global variable ListenSpec.

Change-Id: I885ba57243d89351a5889c050d4d1c5ba9cadd07

vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref,vanadium/go.ref