Make it easier to construct CNAME packets

[maxhawkins]

Summary

Add helper functions to make it easier to construct SourceDescription CNAME packets.

• Make easier to use constructor for SourceDescriptions
• Add helpers for choosing RFC 7022 CNAMEs

Motivation

Currently it's cumbersome to generate CNAME packets:

cname := &SourceDescription{
	Chunks: []SourceDescriptionChunk{{
		Source: 1234,
		Items: []SourceDescriptionItem{{
			Type: SDESCNAME,
			Text: "cname",
		}},
	}},
}

Because every CompoundPacket requires a SourceDescription with a CNAME, we should make creating these packets easy.