flock, fcntl record locks

There were none: fcntl answered F_SETLK with EINVAL and flock was not a
system call. Alpine's apk, which runs unmodified on SchOS through
Alpine's own musl, locks its database before reading it and stopped
there.

kernel/fs/lock.c keeps one table of advisory locks keyed by (dev, inum):
flock() for the whole file, owned by the open file description;
F_SETLK/F_SETLKW/F_GETLK for byte ranges, owned by the process and
released when it closes any descriptor on the file or exits; and the
F_OFD_* forms, ranges owned by the description. flock and record locks
do not see each other, as on Linux. Waiters sleep until any release and
are interrupted by a signal.

cmd/locktest checks each between a parent and a child, including range
splitting, F_GETLK's report, and a blocked waiter being woken.

shell.nix gains pkg-config, meson, ninja, m4, bison, flex and xz, for
the X ports.

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

shell.nix

@@ -38,6 +38,18 @@ pkgs.mkShell {
     xorriso             # make iso
     mtools              # ESP staging
     curl gnutar         # fetching ports
+    xz                  # ...some of which are .tar.xz
+
+    # The X ports (docs/plan/xorg.md): half of them configure with autotools
+    # and half with meson, and all of them ask pkg-config for each other.
+    # libxcb generates its protocol code with python; libX11 and xkbcomp need
+    # m4 and bison at build time.
+    pkg-config
+    meson
+    ninja
+    gnum4
+    bison
+    flex
   ];
 
   shellHook = ''