The first thing aug_load does is delete /augeas/files and /files. We need
to make sure that any variables that contain references to nodes in those
subtrees are cleaned up properly, i.e. that we remove such nodes from
nodesets in the variables.

  * src/augeas.c (tree_unlink_children): remove descendants of the tree we
    are deleting from variables.
  * tests/test-load.c (testLoadDefined): verify fix

Fixes bug #79