Directly install from patches and avoid slackpkg.

[aclemons]

We can fetch the patches directly instead of first installing the packages and then potentially upgrading them.

I have also made it possible to install things from extra - older versions of slackware had slackpkg in extra instead of the ap series.

I dropped the bind mounting of resolv.conf too since nothing needs network access in the chroot now.

[aclemons]

I have another patch to make building slackwarearm / slackwareaarch64 work (and use them to make a 3 arch multi-arch image). But I pulled these changes out first to see what you think of them. If these are fine, I will open a PR for the arm changes too.