Start implementing the "resdevice" (resident device) keyword. It
corresponds to the st_dev field of stat(2), e.g. the ID of the
device that contains the inode.

Change parse_device() prototype so it can be used for "device"
and "resdevice", and call the appropriate archive_entry_set_{,r}dev()
function to store the field value.
Small typo fix in parse_device()'s description.
Implement "resdevice" and "inode" in the mtree(5) spec.
- resdevice is the keyword that corresponds to the "resident" device,
e.g. the one where the entry resides;
- inode is entry's inode number.
Document "resdevice" and "inode".