support multi-partitioned systems

[kazuho]

Unco uses link(2) to create backups when link(2) replaces / unlink(2) removes an existing file (note: the cannot be implemented using copy, since the file being replaced / removed might be linked from another location and it is essential to restore that state).

However link(2) cannot link files on different partitions. So it is better to create a backup directory on every partition, and use the appropriate one to store a backup.