Add "--nopost" option to disable post hooks and use it for kernel-install

Quick idea while I'm around:

Kernel-install already provides a framework for running things pre and post, so it doesn't really make any sense to duplicate the functionality, does it?

For example, with the current behavior, programs like sbctl are called twice by default, once from post hooks and once from kernel-install.

This might break someone's setup who expects post hooks to run even if using kernel-install, so it should probably be announced properly.