Coalesce Secret Key Encryption summary to a single table

This also makes the table a registry -- if someone wants to try to define a new S2K Usage octet, they'll need to register it here.

This makes no substantive change to the text of the document, but it does create a new IANA registry by creating a table labeled "registry".

Without this change, the "S2K usage octet" is not defined in any registry.