Add lockout-related performance tuning variables

Add lockout-related performance tuning variables

The account lockout feature of krb5 1.8 came at a cost in database
accesses for principals requiring preauth, even if lockout is not
used.  Add dbmodules variables disable_last_success and
disable_lockout for the DB2 and LDAP back ends, allowing the admin to
recover the lost performance at the cost of new functionality.

(Unrelated documentation fix: document database_name as a DB2-specific
dbmodules variable instead of the realm variable it used to be.)

ticket: 6719

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@24003 dc483132-0cff-0310-8789-dd5450dbe970