Add another transfer mode --clone to pg_upgrade (besides the existing
--link and the default copy), using special file cloning calls.  This
makes the file transfer faster and more space efficient, achieving
speed similar to --link mode without the associated drawbacks.

On Linux, file cloning is supported on Btrfs and XFS (if formatted with
reflink support).  On macOS, file cloning is supported on APFS.
Reviewed-by: Michael Paquier <michael@paquier.xyz>