net: unbreak ping against hosts that intermittently drop ICMP

Pinging sl0p.foo through gbhub looked hung: DNS resolved, then nothing.
Packet-tracing showed the echo request leaving the hub's TUN and eth0
correctly NATed every time - but 80.78.19.56 blackholes ICMP for all
ids in windows of tens of seconds (provider rate limiting). One lost
reply wedged ping for minutes because net_recv's pump-counted timeout
is effectively unbounded at native emulation speed.

Two fixes:

- NET_RECVNB (op 8): non-blocking recv - one RX pump, $FE if nothing
  buffered, same delivery/EOF semantics as NET_RECV otherwise. ping now
  waits <=~2s per seq (net_recv_nb + msleep loop), prints 'seq=N
  timeout' and moves on, like real ping.

- ICMP echo id was hardcoded $1234 for every GB, every boot, so all
  sessions produced byte-identical flows - hostile to NAT conntrack
  (keyed on icmp id). wNetEchoId is now our host octet + rDIV timing
  noise sampled at DHCP lease, distinct per GB and per boot.

Verified: 8 back-to-back native-speed gbhub runs, zero hangs; a run
that hit a blackhole window printed seq=1 timeout then recovered to
3/4 received. GB<->GB ping and DNS/DHCP unaffected.

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