Tolerate device-id changes (due to automounter) amid copying

[gpziemba]

cpdup examines st_dev (inode device id) of source and destination
file tree objects to detect when a directory descent crosses a
mount point, so it can prune the copy at that point.

Filesystems mounted by the automounter can be automatically
unmounted and re-mounted during the copy operation, changing
the inode device ID for all of the filesystem's objects.

This commit adds a consolidated mechanism for

1. caching the inode device IDs of the roots of the source and destination trees; and

2. comparing inode device IDs to the root's ID as the copy proceeds over the source and destination trees.

If a comparison fails, the root inode device ID is refetched and re-compared before declaring a mount point crossing.

[liweitianux]

Thank you for the patch. We'll have a review and handle it accordingly.