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/core/proc.h

@@ -174,6 +174,7 @@ struct procinfo {
   uint64 nvcsw, nivcsw;     // voluntary / involuntary context switches
   char   name[16];
   char   exe[64];           // full path, for /proc/<pid>/exe
+  int    kthread;           // a kernel thread, not a program
 };
 
 struct proc {
@@ -321,6 +322,14 @@ struct proc {
   // chroot(2): where an absolute path starts, and where ".." stops. 0 is the
   // system's root. Inherited across fork, held as a reference like cwd.
   struct inode *root;
+
+  // Syscall tracing: every call and its result go to the log. Set by exec
+  // when the program's name is the one written to /proc/strace, and kept
+  // across fork so that what a traced program starts is traced too.
+  int strace;
+
+  // umask(2). Inherited across fork; 022 for the first process.
+  uint umask;
 };
 #define FD_CLOEXEC 1