CVE-2020-25722 s4/dsdb/samldb: check for clashes in UPNs/samaccountnames

We already know duplicate sAMAccountNames and UserPrincipalNames are bad,
but we also have to check against the values these imply in each other.

For example, imagine users with SAM account names "Alice" and "Bob" in
the realm "example.com". If they do not have explicit UPNs, by the logic
of MS-ADTS 5.1.1.1.1 they use the implict UPNs "alice@example.com" and
"bob@example.com", respectively. If Bob's UPN gets set to
"alice@example.com", it will clash with Alice's implicit one.

Therefore we refuse to allow a UPN that implies an existing SAM account
name and vice versa.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14564

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
