determine if the build will be a pkgng build or a pkg_install build base on
bsd.port.mk, not on /etc/make.conf, this allows build to autodetermine that on
current the default is pkgng

Tested by:	erwin
Actually use first argument and not scriptname
for figuring out which arch to run on.
- Make sure /dev/fd get first umounted.
- workaround bug leaking user 5201 for unknown reasons

Submitted by:	bapt
- tell jexec whish shell to use
(this seems to fix jexec: execvp(): /buildscript: No such file or directory)
- hardcode zbase to fix client setup.