Fixes & improvments for using homed-luks on 4k disks (#35776)

Mostly consists of fixes to

- use the same sector_size as the fdisk context we are using, when
converting between sectors returned by libfdisk to bytes. Fixes #30394 ,
Fixes #30393
- Use the explicit sector size if specified in the home record when are
probing the image file using libblkid. Fixes #30393

Also contains some other improvements with using physical block devices.

- Automatically probe sector size of physical block device, if user does
not pass luks-sector-size explicitly.
- Assign partitions to 1 MiB boundaries, as it is the standard practice
followed by all tools, fdisk, gptfdisk, gnu parted etc.
- Avoid stacking of loop device on top of physical block device in
home_create_luks as it leads to degradation of discard operations, and
mkfs getting stuck.