Even with DEINSTALL_SRC set empty, changes in the pkgsrc infrastructure over
the past few years will now create a dynamic +DEINSTALL script regardless,
and while only containing "exit 0", may have a #! using a shell from pkgsrc.
This can lead to upgrade issues if that shell happens to be unavailable
while pkg_install is being upgraded. Creating our own dummy script that
explicitly uses /bin/sh avoids that problem.
