requires the return value of telldir() to equal the value passed to
seekdir(). The current seekdir code with SINGLEUSE enabled breaks
this case as each call to telldir() allocates a new cookie. Instead,
remove the SINGLEUSE code and change telldir() to look for an existing
cookie for the directory's current location rather than always creating
a new cookie.
CR: https://phabric.freebsd.org/D490
PR: 121656
Reviewed by: jilles
MFC after: 1 week