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/net.c

@@ -292,12 +292,19 @@ net_thread(void)
     // One bit per interface, so a second one binding later is announced too.
 
 
+    uint link_checked = 0;
     for (;;) {
         int moved;
 
         lwip_sys_lock();
         moved = net_eth_poll();
         sys_check_timeouts();
+        // A quarter of a second is quick enough to notice a cable, and a
+        // register read per interface that often costs nothing.
+        if (ticks - link_checked >= 25) {
+            link_checked = ticks;
+            net_eth_link_check();
+        }
         // Loopback has no driver and no interrupt: a packet sent to 127.0.0.1
         // sits on the loopif's queue until something drains it. With NO_SYS=1
         // that something is this thread.