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/pty.h

@@ -0,0 +1,41 @@
+#pragma once
+
+// Pseudo-terminals, Unix98 style: open /dev/ptmx and get the master side of a
+// new pair; the slave is /dev/pts/N. What sits between the two is the same
+// line discipline a console has. See kernel/dev/pty.c.
+
+#include "core/types.h"
+
+#define PTMX_MAJOR  13      // /dev/ptmx: opening it allocates a pair
+#define PTM_MAJOR   14      // a master, minor = pair number; never in a listing
+#define PTS_MAJOR   15      // /dev/pts/N
+#define NPTY        32
+
+// A controlling terminal that is a pty, in proc->ctty: PTY_CTTY + N. The
+// console's are 1..NVT.
+#define PTY_CTTY    64
+
+struct inode;
+struct file;
+
+int  pty_alloc(void);                // the new pair's number, or -1
+int  pty_used(int n);
+int  pty_ioctl(struct inode *ip, uint request, addr_t arg);
+int  pty_poll(struct inode *ip, int events, int readable, int writable);
+struct inode *ptmx_open(struct inode *ip);  // ip locked; returns the master, locked
+void pty_init(void);
+
+// Lines the devsw table needs.
+int  ptm_read(struct inode*, uint64, char*, int);
+int  ptm_write(struct inode*, uint64, char*, int);
+int  ptm_ready(struct inode*);
+void ptm_open(struct inode*);
+void ptm_close(struct inode*);
+int  pts_read(struct inode*, uint64, char*, int);
+int  pts_write(struct inode*, uint64, char*, int);
+int  pts_ready(struct inode*);
+void pts_open(struct inode*);
+void pts_close(struct inode*);
+
+// From kernel/fs/devfs.c: the inode of master N.
+struct inode *devfs_pty_master(int n);