netmon provides ipv6 availability during network event changes, so use
this metadata instead of wasting on polling check.
Further, repeated network errors will force marking ipv6 as disable if
were being enabled, catching a rare case when ipv6 were disabled from
cli or system settings.
A backoff with small max time will flood requests to Control D server,
causing false positive for abuse mitiation system. While a big max time
will cause ctrld not realize network change as fast as possible.
While at it, also sync DoH3 code with DoH code, ensuring no others place
can trigger requests flooding for ipv6 probing.
In split mode, the code must check for ipv6 availability to return the
correct network stack. Otherwise, we may end up using "tcp6-tls" even
though the upstream IP is an ipv4.
