build - add installworld-force target and better checks
* Add better checks to installworld. Print an error and exit if the
base system is too old, directing users to run installworld-force.
* Add installworld-force. This automates installing the world on a
base system which is too old and thus has chicken-and-egg problems
with binaries and libraries.
This install target will install to a temporary directory, compile up
a static cpdup, and then use it to just blast copy /bin, /sbin, /lib,
/libexec, /usr/bin, /usr/sbin, /usr/lib, and /usr/libexec. It will
then execute a normal make installworld and make upgrade.