Improve CLAT IPv6 address auto-generation logic

In the case of there being more than one EUI-64 based IPv6 address on
the PLAT device, clatd will now pick the one which share the longest
common prefix length with the PLAT prefix when deciding which one to
base the auto-generated CLAT IPv6 address on. This should avoid
accidentally ending up with a ULA-based CLAT IPv6 address when better
alternatives exist.

Resolves #1.