swarm/api/http: correct leaf links when navigating with bzz-list

[acud]

using bzz-list to navigate manifests through the browser does not yield the correct links for viewing the leaf nodes.
example:

https://swarm-gateways.net/bzz-list:/theswarm.eth/Swarm_files/
analytics.js hyperlink shows up as
https://swarm-gateways.net/bzz-list:/theswarm.eth/Swarm_files/analytics.js
instead of:
https://swarm-gateways.net/bzz:/theswarm.eth/Swarm_files/analytics.js

[cobordism]

while on the topic, bzz-list output should make it clear when a manifest has a default (no-name / empty path entry) and offer a clickable link here too.

c.f. #156 (comment)