Enhancement: Restore files with many zeros as sparse files

On all platforms except Windows, the restorer may now write files containing
long runs of zeros as sparse files (also called files with holes): the zeros
are not actually written to disk.

How much space is saved by writing sparse files depends on the operating
system, file system and the distribution of zeros in the file.

https://github.com/restic/restic/issues/79
https://github.com/restic/restic/pull/2601
https://forum.restic.net/t/sparse-file-support/1264