MFC: r234313,234315,234322,234351,234870,235726,238461
add usr.sbin/pkg which is a bootstrap tool for pkgng.
it respects PACKAGESITE, PACKAGEROOT, and a new environment variable ABI (if a
user want to use a different API from the base one for its packages)
it has no man page on purpose to avoid hidding the pkg(8) man page from the
pkgng package.
for now uses pkgbeta.FreeBSD.org as default mirror to find its package
it respects MK_PKGTOOLS
git-svn-id: svn://svn.freebsd.org/base/stable/8@239563
ccf9f872-aa2e-dd11-9fc8-
001c23d0bc1f