Add a metadata() method to Node, File, Symlink, and Dir VFS objects.

Add a metadata() method to Node, File, Symlink, and Dir VFS objects.

The current implementation just populates the self._metadata field of
all of the objects in a directory (and of directory itself) from the
relevant .bupm file whenever metadata() is called on any one of those
objects.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Reviewed-by: Zoran Zaric <zz@zoranzaric.de>
Reviewed-by: Gabriel Filion <lelutin@gmail.com>