add support for captive URL detection via DHCP option 114 venue-info-url

[monperrus]

I've been happily using captive-browser for many years. Sometimes, it does not work because the captive portal advertises the portal URL via DHCP option 114 venue-info-url.

It would be great to support DHCP option 114 venue-info-url.

Documentation:

• https://datatracker.ietf.org/doc/html/draft-ietf-capport-rfc7710bis-11
• https://developer.android.com/about/versions/11/features/captive-portal

Thanks!

[Zocker1999NET]

to refer to newer RFCs which describe how Captive Portals should (hopefully) work in the future:

• RFC 8952: Architecture
• RFC 8908: API
  • machine-readable state of captivity
    • Internet connection currently allowed?
    • if so, how many seconds and/or bytes are remaining?
    • Can user further extend session?
  • separate URIs for
    • captive portal (where user can extend session & etc.)
    • venue info (where user finds train/flight status, entertainment, …)
  • allows for proper TLS for Captive Portals
• RFC 8910: API / portal identification via DHCP & RAs