Add a note about how to apply/remove policies

Put a note in the the policies section of the documentation for how to
apply policies to principals.

[kaduk@mit.edu: reformat commit message]

ticket: 7693 (new)
Remove unused _gssd_keyring_ids_ feature

This feature was intended to be used by gssd to access users' keyring
credentials, but it was never used.

[ghudson@mit.edu: clarified commit message]
Remove unused counter in keyring ccache

numkeys is never really used in the single cache data structure.
Every time a new iteration is started, numkeys is recalculated anyway,
and then only the copy held in the cursor is used.  Remove it from the
cache data and keep it only in the cursor.

[ghudson@mit.edu: clarified commit message]
Simplify krb5_krcc_start_seq_get

This code can be simplified (and a potential race avoided) by using
keyctl_read_alloc() and letting it allocate the necessary memory.
This also allows to remove a helper function that is not used anymore
as well as make the code more readable.  The only penalty is that we
have two allocations instad of one.

[ghudson@mit.edu: trivial simplifications]