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

tools/posix/mkaroot.sh

@@ -234,6 +234,12 @@ ROOT_PI=$ROOT sh -c '
     done
     grep -qw schos "$ROOT_PI/etc/hosts" || printf "127.0.1.1\tschos\n" >> "$ROOT_PI/etc/hosts"'
 
+# The shared scratch directories, world-writable and sticky as baselayout
+# means them. apk running as an ordinary user on the build host lands them as
+# 0755, and with the kernel reporting real modes that is what tmux and every
+# other program checking /tmp would see.
+chmod 1777 "$ROOT/tmp" "$ROOT/var/tmp"
+
 # The repository goes in too, so the installed system can reinstall and
 # upgrade schos-base, and Alpine's are listed for everything else.
 mkdir -p "$ROOT/var/lib/schos"