Demand paging, the RTL8168H, links that come up late, and Ctrl+Alt+F1..F12

- Anonymous memory can be reserved without being backed: large mappings,
  MAP_NORESERVE and PROT_NONE are recorded and each page appears, zeroed,
  on first touch -- from trap() for the program, from uva2ka() for the
  kernel on its behalf. WebKit reserves 4 GB at start and touches a sliver;
  backing it all failed with ENOMEM and Epiphany aborted.
- r8169: the 8168G generation and later (the laptop's 8168H, XID 541) are
  taken from the firmware's out-of-band mode, get their PLL powered, the
  receive gate opened, and their PHY reached through the OCP window. The
  PHY's power-down bit is cleared; the old code kept it while restarting
  autonegotiation. Untested on the hardware so far.
- The network thread follows each device's link into lwIP. The link was
  read once at attach, 0.02 s into boot; real autonegotiation takes
  seconds, so on hardware the interface stayed down for good.
- Twelve terminals, switched with Ctrl+Alt+F1..F12 by default: the keys X
  uses, so the console and the desktop agree. AltGr+F-key is gone from
  the defaults (chvt -c altgr+F brings it back).
- SO_PASSCRED on Unix sockets, with SCM_CREDENTIALS delivered by recvmsg.
  WebKit sets it on its IPC sockets and aborted when refused.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/net/lwip/netif_eth.c

@@ -259,3 +259,28 @@ net_eth_poll(void)
     }
     return moved;
 }
+
+// Follow each device's link into lwIP.
+//
+// The link was read once, at attach, and never again. Under QEMU it is up
+// before the driver asks; on a real network autonegotiation takes two or
+// three seconds, so attach saw it down and the interface stayed down for
+// good -- no DHCP, no traffic, the cable plugged in and nothing to show for
+// it. Bringing the link up also restarts DHCP (lwIP does that itself).
+void
+net_eth_link_check(void)
+{
+    for (int k = 0; k < nifs; k++) {
+        struct eth_if *e = &ifs[k];
+        if (!e->used)
+            continue;
+        int up = netdev_link_up(e->dev) != 0;
+        if (up == (netif_is_link_up(&e->netif) != 0))
+            continue;
+        if (up)
+            netif_set_link_up(&e->netif);
+        else
+            netif_set_link_down(&e->netif);
+        klog("net: %s link %s\n", e->dev->name, up ? "up" : "down");
+    }
+}