s4-dsdb Reorganise and clarify the LSA objectClass check (forbidden on LDAP)

s4-dsdb Reorganise and clarify the LSA objectClass check (forbidden on LDAP)

This arranged the check to avoid talloc_strdup() (the schema pointers
are constant, and can be relied upon), and checks the untrusted bit
first (it is faster), before the ldb_attr_cmp().

The strcmp() here was valid, if unusual, because the ldapDisplayName
values are already in the correct case, but strcasecmp() is more
correct, as for the small extra cost, we avoid a difficult to diagnose
bug later.

Andrew Bartlett

Signed-off-by: Matthias Dieter Wallnöfer <mdw@samba.org>