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
Approved by: des (mentor)
