Add two new parameters to control how we verify kerberos tickets. Removes lp_use_kerb...

Add two new parameters to control how we verify kerberos tickets. Removes lp_use_kerberos_keytab parameter.

The first is "kerberos method" and replaces the "use kerberos keytab"
with an enum.  Valid options are:
secrets only - use only the secrets for ticket verification (default)
system keytab - use only the system keytab for ticket verification
dedicated keytab - use a dedicated keytab for ticket verification.
secrets and keytab - use the secrets.tdb first, then the system keytab

For existing installs:
"use kerberos keytab = yes" corresponds to secrets and keytab
"use kerberos keytab = no" corresponds to secrets only

The major difference between "system keytab" and "dedicated keytab" is
that the latter method relies on kerberos to find the correct keytab
entry instead of filtering based on expected principals.

The second parameter is "dedicated keytab file", which is the keytab
to use when in "dedicated keytab" mode.  This keytab is only used in
ads_verify_ticket.