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.

1 files changed, 6 insertions, 0 deletions

diff --git a/c/sock.h b/c/sock.h
index 9e4dd40..876e85c 100644
--- a/c/sock.h
+++ b/c/sock.h
@@ -42,6 +42,12 @@ static unsigned char net_recv(unsigned char s, void *buf, unsigned char max) {
     _nr.op = 3; _nr.sock = s; _nr.buf = (unsigned char *)buf; _nr.len = max;
     return sys_net(&_nr);
 }
+/* non-blocking recv: one RX pump -> length, 0xFE if nothing buffered yet,
+   0 on TCP EOF. Loop it with msleep() to own your timeout (see ping). */
+static unsigned char net_recv_nb(unsigned char s, void *buf, unsigned char max) {
+    _nr.op = 8; _nr.sock = s; _nr.buf = (unsigned char *)buf; _nr.len = max;
+    return sys_net(&_nr);
+}
 /* set the local (source) port - needed so UDP replies demux back to us */
 static unsigned char net_bind(unsigned char s, unsigned int port) {
     _nr.op = 5; _nr.sock = s;