net, rpc: return NET_UNROUTABLE as not_publicly_routable, automate helps

Summary:

net: update NET_UNROUTABLE to not_publicly_routable in GetNetworkName()

net: create GetNetworkNames()

rpc: use GetNetworkNames() in getnetworkinfo and getpeerinfo helps

init: use GetNetworkNames() in -onlynet help

This is a backport of core#20965

Depends on D11030

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11036