If telldir() is called immediately after a call to seekdir(), POSIX
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