Make restore overwrite behavior configurable

[MichaelEischer]

Restore should implement additional options to make it more flexible. This is a tracking issue that sums up several existing issues.

New overwrite behaviors

Skip existing:

• --ignore-existing
• does not overwrite already existing files

Always overwrite (default):

• --delete option to remove files not in the snapshot
• will also overwrite readonly files etc.
• if a file would overwrite a non-empty directory, refuse to do so unless --delete is also specified. That semantic is also used by rsync

Overwrite if newer:

• --overwrite-if-newer (rsync --update) replaces local files if the snapshot content is newer (by file timestamp!)
• allow implementing some kind of sync
• --delete option works the same way as for "Always overwrite"

Optimizations:

• restore only changed files: user can select to trust timestamp+size (default) or to let restic check the file contents of everything

Addresses Issues:

• Feature request: "Restore newer/missing datasets" / "synchronize" #200 Feature request: "Restore newer/missing datasets" / "synchronize"
• Make restore resumeable. #407 Make restore resumeable.
• Option for restic restore to delete local files not in snapshot #2348 Option for restic restore to delete local files not in snapshot
• Restore only changed files #2662 Restore only changed files
• Restore in place on Windows #4575 Restore in place
• When restoring, cannot rewrite files that are readonly #4757 When restoring, cannot rewrite files that are readonly