buildworld - bootstrap compatibility with older dragonfly's

* Fix buildworld issue bootstrapping grep related utilities which expect
  newer header files.
pkg - Adjust pkg-bootstrap-force

* Adjust the pkg-bootstrap-force target to also remove
  "/usr/local/etc/pkg.conf".  Older pkg.conf files (e.g. 3.5 or earlier,
  possibly 3.6 or earlier) can interfere with newer pkg bootstraps.
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.