Fix restore with --force over existing regular file.

For comparing, check source filesystem's abilities

Clean up compare reports

Test comparing of single files, and files/directories specified by
increment.  Also test --include/--exclude with compare options.

---------[ Medium term ]---------------------------------------

Look into sparse file support (requested by Stelios K. Kyriacou)

Look at Kent Borg's suggestion for restore options and digests.

---------[ Long term ]---------------------------------------

Think about adding Gaudet's idea for keeping track of renamed files.

Look into different inode generation techniques (see treescan, Dean
Gaudet's other post).