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

include/dev/vt.h

@@ -82,7 +82,7 @@ struct vtcell {
 // Escape parser states. E_OSC and E_OSC_ESC exist for hyperlinks: a terminal
 // that does not follow OSC 8 must still consume it, or the URL is printed as
 // text. See the comment in kernel/core/console.c that this came from.
-enum { VE_NORMAL, VE_ESC, VE_CSI, VE_OSC, VE_OSC_ESC };
+enum { VE_NORMAL, VE_ESC, VE_CSI, VE_OSC, VE_OSC_ESC, VE_CHARSET };
 
 struct vt {
   int  index;                       // 0-based; /dev/tty<index+1>
@@ -98,6 +98,10 @@ struct vt {
   // CR, LF, backspace, an escape sequence -- clears it.
   int  wrap_pending;
   int  saved_cx, saved_cy;          // DECSC/DECRC
+  // The scrolling region, DECSTBM: rows scroll_top..scroll_bot inclusive.
+  // scroll_bot 0 means unset, the whole screen -- which keeps a grid resized
+  // underneath it from holding a stale bottom.
+  int  scroll_top, scroll_bot;
 
   uint8 fg, bg;                     // current SGR colours
   int   bold;