kernel: HTTPS through wget, half-close, and /proc/<pid>/fd

busybox wget hands TLS to ssl_client over a socketpair and then does
shutdown(SHUT_WR). That cut the AF_UNIX link both ways, so ssl_client's
reply was a SIGPIPE and every https:// download hung. SHUT_WR is now a
half-close: the peer reads EOF and can still answer. TCP sockets get
shutdown() at all (tcp_shutdown for the FIN); it used to do nothing.

- exec keeps ignored signals and the signal mask, as execve(2) says.
- /proc/<pid>/fd: a link per descriptor -- the path it was opened by,
  socket:[n], pipe:[n], anon_inode:[eventpoll]. /proc/self too. The
  global inode bit moves above pid << 8; at 0x10000 it was /proc/256.
- fstat on a socket or pipe answers S_IFSOCK / S_IFIFO instead of EINVAL.
- poll, select and epoll wake as soon as a pipe, socket or input device
  changes, not on the next tick.
- Pipes are 64 KB of lazily allocated pages, copied in blocks, with
  PIPE_BUF atomicity: 300 MB through `yes | head | wc` in 7 s.

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

kernel/sys/sys_net.c

@@ -656,9 +656,8 @@ sock_name(int peer)
 SYSCALL_DEFINE(getsockname) { return sock_name(0); }
 SYSCALL_DEFINE(getpeername) { return sock_name(1); }
 
-// shutdown(2). Half-close is not modelled -- there is one buffer per
-// direction and no per-direction flag -- so SHUT_WR and SHUT_RDWR both mean
-// "the peer sees end-of-file", which is the behaviour programs depend on.
+// shutdown(2). SHUT_WR is a half-close: the peer reads EOF and can still
+// write back (see sock_shutdown).
 SYSCALL_DEFINE(sockshutdown)
 {
     struct socket *s;