Refactoring/rewrites

New function dentry_tree_fix_inodes() to de-duplicate some code in
read_metadata_resource() and wimlib_add_image().

In wimlib_export_image(), search through the inode list when looking for lookup
table entries to export instead of the dentry tree.

In wimlib_mount_image(), search through the inode list when resolving lookup
table entries, and update the inode numbers at the same time.