loader.efi: efipart does not recognize partitionless disks

[tsoome]

Summary

Rework the block device handle check to allow more robust device
classification.

[tsoome]

efipart_hdd() was quite broken.

[tsoome]

Update to revision 325851.

[imp]

This looks OK, but the code could use some high level comments outlining the algorithms used since they are becoming harder to discern from the just reading the code.

[tsoome]

In D13026#272423, @imp wrote:

This looks OK, but the code could use some high level comments outlining the algorithms used since they are becoming harder to discern from the just reading the code.

Yes, those are waiting in queue - I have some suggestions from illumos review and need to move those over here too.

[tsoome]

Add more comments and reorder some checks