Rename "entries" to "root" to better reflect the future role of
this member.

Modify the bid_mtree behavior so that it can detect correct paths
on the presence of '/' OR '.' (root), which is usually the first
valid entry found in NetBSD's mtree with "form D".
Add a rbnode member for mtree_entry lookup.
Split "next" pointer into "parent" and "next", where "parent" points
to the entry of the parent, and "next" points to an entry positioned
at same depth level in the tree as the entry.
All be used for file hierarchy and/or specifications comparisons (this
matches the logic used for mtree entries with archive_write).
- Enable mtree_verify() so that we can test the archive_read for
  libarchive's mtree.
- Don't forget to free() the archive_read struct used for spec
  parsing.
- Trim whitespace.