Create a function to translate UEFI paths to unix paths

efivar_device_path_to_unix_path translates from UEFI to Unix
efivar_unix_path_to_device_path translates from Unix to UEFI

At present, only HD() device types are supported (both GPT and
MBR). CdRom and floppy devices aren't supported. ZFS isn't supported
because there's no way in the UEFI standard to specify a ZFS datastore.
Network devices aren't supported either.

Three forms of Unix path are accepted: /path/to/file (for a mounted
filesystem), //path/to/file (uses the EFI partition on the same disk
as /), and dev:/path/to/file (for unmounted filesystem). Two forms are
produced (the first and last).

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