pkg_install: Add an explicit dummy DEINSTALL script.

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.


(jperkin)

#!/bin/sh
#
# Dummy script to ensure pkgsrc never creates its own DEINSTALL script for
# this package that uses a pkgsrc shell, as that can cause upgrade issues.
#
exit 0