Consider ejecting pkgadd from the repo

[Garrett D'Amore]

pkgadd (and friends) are no longer used by IPS distros, and -core doesn't "have" packaging. So we should consider ejecting these tools into a separate repo. Others can pick them up from there if they so choose.

[Garrett D'Amore]

Note that pkgadd has hooks into libinstzones and libadm. "Fixing" the former will mean reverting pkgadd support for sparse zones. (Which arguably would be a good thing, but some of the pkgadd users rely on this for reasons that elude me.) Its unclear how hard it would be to pry apart the latter.

However, the priority of this can be reduced now that #63 is resolved.