[bug#27705] [PATCH v4 2/4] build: Allow mounting of entire disks.

[Danny Milosavljevic]

Okay I'm risking it.  A big reason for magic numbers in all the common 
superblock types and also in the partition table is that this usecase of 
optional partition tables works.

I've also excessively tested it.

So I think the risk of breakage is low enough and I've pushed a variant of this 
patch to master as 9833bcfc08ef009b9e8b4398baa481ef65c80ad7.

I'll be obsessively checking the Guix mailing lists through the next few days - 
and if it causes problems I'll revert.  Should I be too slow, feel free to 
revert 9833bcfc08ef009b9e8b4398baa481ef65c80ad7 (only; the other patches of the 
patchset are independent and aren't a cause of concern).

Since the modified code is part of the system generation one can select the 
older generation in the Grub boot menu should the new one not work.