all: implement RFC8910 captive portal (CP) option

all: implement RFC8910 captive portal (CP) option

https://www.rfc-editor.org/rfc/rfc8910.html

RFC8908 defines a captive portal client (CPC) that handles the API URI returned.

The captive portal (CP) option presence signals a portal that requires
authentication beyond what odhcp6c currently handles.

It's possible that a user connecting through an openwrt gateway encounters the
portal anyway (though this behaviour depends on the portal), but if this is not
the case, with this addition we can:
 - surface a message in a UI as to presence of the CP
 - signal that users install a CPC (a chicken/egg problem behind a CP)
 - provide the API URI for downstream consumers.

This should ease the use of travel router scenarios.

Downstream consumers of the API URI can find it via the CAPTIVE_PORTAL_URI
environment/ubus property.

The strengths of having this option handled means downstream consumers get one
unified environment variable since ra.c does not yet handle CUSTOM_* the way
DHCPv6 does.

Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcp6c/pull/127
Signed-off-by: Álvaro Fernández Rojas <[email protected]>