zoneconf.in: Isolate our local variables from the configuration file.

zoneconf.in: Isolate our local variables from the configuration file.

I had a signature failure after changing the configuration file caused
by variables set in the config file interfering with those in the
command procedure.  There are good reasons why `confspc-eval' uses
`uplevel'; but this situation is bad, so it wants fixing properly.

Introduce a new procedure `isolate' which exists largely so its
variables can be clobbered with impunity.