I saw two options for cleaning the env:

 (1) *environ = NULL;
     in zexecve, make setting $_ conditional upon there being any
     environmental variables at all
 (2) pass a new envp around, which if NULL is new environ.

The second is more intrusive but doesn't have an ugly hack and it does
let _exactly_ the environment specified be passed.  I decided that "env"
could easily be a shell builtin if an explicit envp is passed, so
decided in favour of intrusiveness against hackiness.

I think that env-as-builtin is best left pending BINF-handler functions.
So this just leaves the way open.

I know that using a static data-segment variable for an envp is normally
not the best of ideas, I'm relying upon nothing messing with the explict
non-current newenvp before an execve().

-Phil