Log Message:
Modify the pkginstall framework so that it manages all aspects of
INSTALL/DEINSTALL script creation within pkgsrc.

If an INSTALL or DEINSTALL script is found in the package directory,
it is automatically used as a template for the pkginstall-generated
scripts.  If instead, they should be used simply as the full scripts,
then the package Makefile should set INSTALL_SRC or DEINSTALL_SRC
explicitly, e.g.:

	INSTALL_SRC=	${PKGDIR}/INSTALL
	DEINSTALL_SRC=	# emtpy

As part of the restructuring of the pkginstall framework internals,
we now *always* generate temporary INSTALL or DEINSTALL scripts.  By
comparing these temporary scripts with minimal INSTALL/DEINSTALL
scripts formed from only the base templates, we determine whether or
not the INSTALL/DEINSTALL scripts are actually needed by the package
(see the generate-install-scripts target in bsd.pkginstall.mk).

In addition, more variables in the framework have been made private.
The *_EXTRA_TMPL variables have been renamed to *_TEMPLATE, which are
more sensible names given the very few exported variables in this
framework.  The only public variables relating to the templates are:

	INSTALL_SRC		INSTALL_TEMPLATE
	DEINSTALL_SRC		DEINSTALL_TEMPLATE
				HEADER_TEMPLATE

The packages in pkgsrc have been modified to reflect the changes in
the pkginstall framework.