Canonical should accept non-existing paths

[kriskowal]

The reroot implementation assumes that canonical will expand any symbolic links along a path, but will just normalize any trailing terms that do not exist.

Canonical will need to be reimplemented in terms of lstat and realpath.

Alternately, reroot may need to be reimplemented in terms of the existing canonical. reroot needs to be weary of relative paths that would escape the jail.