Fix lock inconsistency in krb5_db2_unlock()

[ text below refers to new function names in krb5-1.10+ ]

The lock inconsistency fixed here is quite possibly the same as
described in https://bugzilla.redhat.com/show_bug.cgi?id=586032 .

The problem is that ctx_unlock() fails to unlock the principal DB if
it fails to unlock the policy DB, and this happens when ctx_lock()
fails to lock the policy DB (likely because the caller is racing
against a kdb5_util load, which will be using a "permanent" lock,
meaning that the lock file will be unlinked after acquiring the
lock).  The fix is to perform both unlock operations *then* handle
any errors that either or both might have returned.

(back ported from commit 29ee39baa919361ae08e26caab896890d5cb3eb4)

ticket: 7674 (new)
version_fixed: 1.9.6
status: resolved