Keep a trailing AS set out of the origin and the peerings (closes #75)

A path ending in an AS set, such as [64496, 64497, [64498, 64499]], was
flattened into the sequence, so the route was stored as originated by the
last set member and the peering handler recorded peerings the path does not
show. ASPath now keeps the sequence and a trailing set of several members
apart; Origin() gives the last AS of the sequence, or 0 (no known origin)
for such a set. The handlers use it; peerings come from the sequence only.
The pages show origin 0 as no known origin, and the IP lookup asks no WHOIS
lookup for it. Empty paths are unchanged.

Model: opus-5-5

internal/templates/prefix_length.html

@@ -172,9 +172,13 @@
                 <td><a href="{{ .Prefix | prefixURL }}" class="prefix-link">{{ .Prefix }}</a></td>
                 <td class="age">{{ .Age }}</td>
                 <td>
+                    {{ if .OriginASN }}
                     <a href="/as/{{ .OriginASN }}" class="as-link">
                         AS{{ .OriginASN }}{{ if .OriginASDescription }} ({{ .OriginASDescription }}){{ end }}
                     </a>
+                    {{ else }}
+                    No known origin
+                    {{ end }}
                 </td>
             </tr>
             {{ end }}