[WIP] module-setup.sh: catch installation failures, warn or fail as appropriate

[ahesford]

By default, check now always returns 255 to prevent automatic installation of the dracut module; it must be explicitly requested or pulled in as a dependency.

install now checks the exit codes of all critical installation commands and, for files that absolutely must exist in the initramfs, failure to install will trigger a dracut failure. For a few optional files, warnings are emitted but installation continues. We can argue about the specific "essential" executables, but most are required in the early parts of zfsbootmenu.sh that will get to the point of automatically booting a kernel and, for some others like grep, they should definitely be in the image to ensure that the emergency shell has important functionality in the event the menuing system cannot be displayed or some other critical error occurs. (These commands should be sufficient to at least set a bootfs or, for those with a deeper understanding, drive the ZBM kexec sequence manually---this has saved me a few times!)

When we merge skim_support, fzf will have to be special-cased to fallback to sk and, if that isn't available, emit the warning.

I'm marking WIP because this is totally untested but looks functional and because I want to get some feedback before we really dive in here.

NB: I've dropped the /usr/bin prefix from the ZFS executables. I'm not sure why they were there in the first place, but it's another point that needs testing.

[zdykstra]

With the exception of what should be considered required and optional, I like this. It does highlight other bits of hardening we should/could do - but aren't strictly in the scope of this PR.

[ahesford]

Still untested, but ready for testing. I put the sk fallback install in here even though we still need the additional logic from skim_support; there were going to be merge conflicts no matter what.

I added blkid as a hard requirement. It seems to be in there as a consequence of udev, but since we call it explicitly in the check for resume images, we should explicitly require it.

[zdykstra]

I added blkid as a hard requirement. It seems to be in there as a consequence of udev, but since we call it explicitly in the check for resume images, we should explicitly require it.

Good catch on blkid !

[zdykstra]

I've tested creating a few different initramfs's that check the major code paths in module-setup.sh. A full clone test in the initramfs works as well.