Fix a few bugs, some of which I introduced in recent commits:

Fix a few bugs, some of which I introduced in recent commits:
- clean_mtab():
    Actually use the strdup'd version of the host that we go to the
    trouble of creating.
- do_umntall/do_umount:
    Don't return success if clnt_create() fails.
    Don't access a client pointer after it has been destroyed.
    Remember to destroy the authentication information we created.