* [proxy] Add --external-ip-map flag for the embedded client
The proxy behind a NAT with a forwarded fixed --wg-port could not advertise
its public address as an ICE candidate, so peers fell back to relay. Expose
the agent's external IP map through embed.Options and a proxy flag
(NB_PROXY_EXTERNAL_IP_MAP).
* [proxy] Validate --external-ip-map at startup
The engine drops every external IP mapping when one fails to parse, so a
typo or a space after a comma silently disabled the feature. Move the
agent's validator to shared/externalipmap, run it from the proxy at
startup, and trim entries first.
* [client,proxy] Require an external IP first in --external-ip-map
The engine reads the first component as the external IP, so a mapping like
iface/10.0.0.5 passed validation and then dropped every mapping.
Nicolas Frati
357d8a2a7c