CVE-2020-10745: ndr/dns-utils: prepare for NBT compatibility

CVE-2020-10745: ndr/dns-utils: prepare for NBT compatibility

NBT has a funny thing where it sometimes needs to send a trailing dot as
part of the last component, because the string representation is a user
name. In DNS, "example.com", and "example.com." are the same, both
having three components ("example", "com", ""); in NBT, we want to treat
them differently, with the second form having the three components
("example", "com.", "").

This retains the logic of e6e2ec0001fe3c010445e26cc0efddbc1f73416b.

Also DNS compression cannot be turned off for NBT.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14378

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>