Enhancement: Restore files with many zeros as sparse files

When using `restore --sparse`, 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.

During backup restic still reads the whole file including sparse regions. We
have optimized the processing speed of sparse regions.

https://github.com/restic/restic/issues/79
https://github.com/restic/restic/issues/3903
https://github.com/restic/restic/pull/2601
https://github.com/restic/restic/pull/3854
https://forum.restic.net/t/sparse-file-support/1264