"UNKNOWN" is not a good name. If you don't set the property
in the connection explicitly, it should be "DEFAULT".

Also, make "DEFAULT" -1. For one, that ensures that the enum's
underlying integer type is signed. Otherwise, it's cumbersome
to test "if (mdns >= DEFAULT)" because in case of unsigned types,
the compiler will warn about the check always being true.
Also, it allows for "NO" to be zero. These are no strong reasons,
but I tend to think this is better.

Also, don't make the property of NMSettingConnection a CONSTRUCT property.
Initialize the default manually in the init function.

Also, order the numeric values so that DEFAULT < NO < RESOLVE < YES with
YES being largest because it enables *the most*.