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/unix.c

@@ -271,6 +271,7 @@ sockopt_set(struct socket *s, int level, int name, int val)
     case SO_REUSEADDR: s->o_reuseaddr = !!val; break;
     case SO_KEEPALIVE: s->o_keepalive = !!val; break;
     case SO_BROADCAST: s->o_broadcast = !!val; break;
+    case 16:           s->o_passcred  = !!val; break;     // SO_PASSCRED
     // Accepted and left at the size we have. Not the same lie as before:
     // Linux is explicitly free to clamp these, and sockopt_get below reports
     // the real figure rather than the requested one.
@@ -313,6 +314,7 @@ sockopt_get(struct socket *s, int level, int name, int *out)
     case SO_REUSEADDR:  *out = s->o_reuseaddr; break;
     case SO_KEEPALIVE:  *out = s->o_keepalive; break;
     case SO_BROADCAST:  *out = s->o_broadcast; break;
+    case 16:            *out = s->o_passcred;  break;     // SO_PASSCRED
     // The real buffer, not a number somebody asked for.
     case SO_SNDBUF:
     case SO_RCVBUF:     *out = SOCKBUF; break;
@@ -1025,6 +1027,12 @@ socksendto(struct socket *s, char *buf, int n, uint addr, ushort port)
   return sockwrite(s, buf, n);
 }
 
+int
+sock_passcred(struct socket *s)
+{
+  return s->domain != AF_INET && s->o_passcred;
+}
+
 // SO_PEERCRED: the credentials recorded for the far end of a connected Unix
 // socket. -1 if there is no far end.
 int