[Bug] wrapper.c uses unportable unsetenv

The unsetenv()/setenv(overwrite) calls are not 100% portable - as in not on
all POSIX implementations. It breaks the build on some of the NonStop
platforms. This will change in a year or two but I really don't want to fall
behind on git releases.

This was introduced at 3540c71 but I was on vacation when it happened so did
not catch it during reviews - my apologies for that.

Is it critical that this be called or can we #ifdef it away if it isn't
supported for a build? The #if is exactly this:

wrapper.c@150
+ #if (_TANDEM_ARCH_ > 3 || (_TANDEM_ARCH_ == 3 && __L_Series_RVU >= 2010))
	if (setenv(name, value, overwrite))
		die_errno(_("could not setenv '%s'"), name ? name :
"(null)");
+ #endif

wrapper.c@154
+ #if (_TANDEM_ARCH_ > 3 || (_TANDEM_ARCH_ == 3 && __L_Series_RVU >= 2010))
	if (!unsetenv(name))
		die_errno(_("could not unsetenv '%s'"), name ? name :
"(null)");
+ #endif

-Randall