Pseudo-terminals, and what tmux and htop needed around them

- /dev/ptmx and /dev/pts/N: Unix98 pairs with a full line discipline,
  window sizes, job control and hangup (kernel/dev/pty.c).
- The console speaks scroll regions, insert/delete line, SU/SD, RI/IND/NEL,
  DECSC/DECRC; swallows charset designations and private CSI sequences;
  and LF only returns the carriage under ONLCR.
- setsid() drops the controlling terminal, so a pane's pty becomes its ctty.
- pipe2() honours O_NONBLOCK and O_CLOEXEC (libevent's signal pipe).
- File modes are stored and enforced: chmod, fchmod(at), umask, creation
  modes; /tmp is 1777.
- /proc/stat has real user/system/idle; /proc/<pid>/task; kthread flag.
- SO_PEERCRED, IOV_MAX 1024.
- /proc/strace: name a program and its syscalls go to the kernel log.
- World gains htop, btop, tmux, mc, ncdu, fastfetch, games, X toys, NetSurf.

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

kernel/core/main.c

@@ -39,6 +39,7 @@ int boot_debug;
 // the VBE info block, not 0x7000 (startothers() below uses that address for
 // the AP boot trampoline).
 #include "arch/schboot.h"
+#include "dev/pty.h"
 
 // Bootstrap processor starts running C code here.
 // Allocate a real stack and switch to it, first
@@ -269,6 +270,7 @@ kmain(uint64 fb_addr)
   // Before the keyboard and mouse, which start producing events the moment
   // their handlers are registered.
   inputinit();
+  pty_init();
   if (boot_verbose) cprintf("verbose boot: inputinit done\n");
   // Before consoleinit, because the console writes to a terminal and there
   // has to be one. vt_init reads the framebuffer geometry, so it also has to