Installer pkgbase support

Release installer artifacts (e.g. disc1.iso) can now be built with
pkgbase packages included, instead of legacy dist sets. This is
enabled by setting -DPKGBASE when building installer images (such as
disc1.iso).

For example:
```
make buildworld buildkernel
cd release
make -DPKGBASE disc1.iso
```

Right now this will still build the legacy dist sets in addition to
pkgbase packages, and include the MANIFEST containing the hashes (but
not the dist sets themselves). The installer prompts for the choice of
legacy sets or pkgbase, and choosing the legacy path will perform an
online installation.

The build time will increase for now, as the work to produce
compressed archives doubles (each file in the base system will be
included in both a legacy dist tarball and pkgbase package).

We'll start producing weekly snapshots with pkgbase packages in the
near future, but images for testing can now be built locally.