feat(cmd_mount): Add support for magic keyword `OLD_BLKID_UUID`

feat(cmd_mount): Add support for magic keyword `OLD_BLKID_UUID`

This commit enhances mount sub command by allowing it to accept
`OLD_BLKID_UUID=xxx` as the device, enabling whole fs mounting without relying
on colon-separated devices.

Using colon-separated devices in fstab is discouraged due to potential device
reference issues after reboot. Additionally, using `UUID=xxx` in fstab or with
`mount -t bcachefs` is currently infeasible as it depends on blkid, which prior
to v2.39, does not recognize bcachefs. This update enables mounting via fstab
using `OLD_BLKID_UUID=xxx` as obtained from `bcachefs show-super` "External
UUID".

Signed-off-by: TruongSinh Tran-Nguyen <i@truongsinh.pro>