Linux: Make keyring destructor remove all tokens

Linux: Make keyring destructor remove all tokens

The keyring destructor on Linux was only destroying the first
unixuser structure found for the PAG being destroyed. In environments
where each PAG contains tokens for multiple cells, this led to us
leaking unixuser structures, and eventually grinding to a hault due
to the length of the resulting hash chains (and some horrific
O(n-squared) performance in the PAG statistics code).

Add a utility function for the keyring destruction function to call
which will mark as deleted all unixuser structures for a particular
PAG, and use it.

Change-Id: I355465c43cc2cf9d70f613d6f97cd65552260157
Cc: Eric.Hagberg@morganstanley.com
Reviewed-on: http://gerrit.openafs.org/1524
Tested-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>