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

include/core/defs.h

@@ -546,6 +546,7 @@ int             sockbind_in(struct socket*, uint addr, ushort port);
 int             sockconnect_in(struct socket*, uint addr, ushort port);
 int             sock_domain(struct socket*);
 int             sock_peercred(struct socket*, int*, uint*, uint*);
+int             sock_passcred(struct socket*);
 // Socket options. The syscall layer does the user-memory marshalling and these
 // do the work, because struct socket's fields live in kernel/net/unix.c and the
 // syscall layer deliberately cannot see them.
@@ -733,6 +734,7 @@ int             net_eth_detach(struct device *);
 int             net_detach(struct device *);
 int             net_attach_late(struct device *);
 int             net_eth_poll(void);
+void            net_eth_link_check(void);
 int             net_eth_count(void);
 void            net_eth_counts(int i, uint64 *rx, uint64 *tx);
 struct netif   *net_eth_netif(int i);
@@ -805,6 +807,8 @@ void            inotify_file(struct file *f, uint32 mask);
 uint32          inotify_cookie(void);
 int             ev_fionread(struct evobj *o);
 int             pipe_avail(struct pipe *p);
+int             vm_fault(addr_t va);
+pte_t*          walkpte(pml4e_t *pgdir, const void *va);
 int             path_identity(char *path, int follow, int onlydir, uint *dev, uint64 *inum);
 int             memfd_rw(int id, int user, uint64 buf, uint64 off, int n, int writing);
 void            shm_detach(int id);