Make cdup return the result of cd.

Just because a matching directory is found, doesn't mean we can
actually enter it. For example, "cdup lost+found" should return and
error code for most users.
Revert "Let cdup switch to children of the current directory."

This reverts commit 40aeeb5c4dbf3170a296101e176038f826202562.

In retrospect, this wasn't a good idea. cdup should always be looking
up.