ILNP addressing using Preference values

The Identifier Locator Network Protocol (ILNP) for IPv6 is described in Experimental RFCs 6740-6744. This document clarifies how addressing in ILNP makes use of Preference values with Locator (L64) and Node Identifier (NID) values. This includes the way that Preference values are used for forming Identifier-Locator Vector (I-LV) values, and selecting I-LVs for use. This document updates RFC6740, RFC6741, RFC6742, RFC6748.