Allow enabling the pkgng version of 490.status-pkg-changes without
also enabling the base system version of 490.status-pkg-changes on 9.x
of earlier, where pkg_tools are installed.
400.status-pkg should be a weekly periodic script

loop rather than repeating essentially the same code
Allow enabling the pkgng version of 400.status-pkg when there are both
pkgng and pkg_tools versions on the same system.
fix #599

Fix up /etc/periodic.conf if requested by use of '-p' flag.  Pull in
the current periodic settings, and update /etc/periodic.conf to
disable any active periodic scripts that depend on pkg_tools(8). Make
a 1-to-1 correspondance between existing pkg_tools periodic jobs, and
pkgng periodic jobs, disable pkg_tools jobs and enable the equivalent
set of pkgng jobs.

Update the 'pkg_version' variable used by weekly_status_pkg_enable if
it still references pkg_version(1)