kcc_utils: Prevent multiple sites attached to a sitelink covering a site

kcc_utils: Prevent multiple sites attached to a sitelink covering a site

This avoids trivial duplicates in a similar manner as mentioned in:

https://blogs.technet.microsoft.com/askds/2011/04/29/sites-sites-everywhere/

It prefers the largest sites then the earliest alphabetically, so that
only a single site ever covers an uncovered site (within a site link).
Note that this isn't applicable over multiple site links (like Windows
presumably) and is only a simple mechanism to avoid excessive
registering.  DCs within the site will also still register for each.

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