Initial commit: Schunix as it stands before the SchOS rename

An x86-64 operating system built from xv6, now largely rewritten: SMP,
a copy-on-write root filesystem (SchFS), PCI/USB/virtio drivers, a TTF
rasteriser, and a Linux-numbered syscall ABI.

No history before this point -- the tree was never under version control.
This commit exists so the rename that follows has something to undo to.

tools/mktree.sh

@@ -0,0 +1,38 @@
+#!/bin/sh
+# Build the synthetic test tree used by schfs_roundtrip.sh into $1.
+# Kept separate so the tree can be recreated for debugging without running
+# the whole round-trip.
+set -e
+TREE=${1:?usage: mktree.sh <dir>}
+rm -rf "$TREE"
+mkdir -p "$TREE/bin" "$TREE/etc/conf.d" "$TREE/empty-dir" "$TREE/a/b/c/d/e/f/g"
+
+: > "$TREE/etc/empty-file"
+echo "hello schfs" > "$TREE/etc/motd"
+printf 'x' > "$TREE/etc/one-byte"
+
+dd if=/dev/zero bs=4096 count=1 2>/dev/null | tr '\0' 'A' > "$TREE/etc/exactly-4096"
+cp "$TREE/etc/exactly-4096" "$TREE/etc/block-plus-one"
+printf 'B' >> "$TREE/etc/block-plus-one"
+
+dd if=/dev/zero bs=1000 count=37 2>/dev/null | tr '\0' 'C' > "$TREE/bin/multiblock"
+cp "$TREE/bin/multiblock" "$TREE/bin/multiblock-twin"
+dd if=/dev/zero bs=1000 count=37 2>/dev/null | tr '\0' 'D' > "$TREE/bin/decoy"
+
+ln -s ../etc/motd "$TREE/bin/motd-link"
+ln -s /nowhere    "$TREE/bin/dangling-link"
+
+echo deep > "$TREE/a/b/c/d/e/f/g/deep.txt"
+echo one  > "$TREE/etc/conf.d/one"
+
+echo spaced > "$TREE/etc/a file with spaces"
+echo dotted > "$TREE/etc/.hidden"
+LONG=$(printf 'n%.0s' $(seq 1 255))
+echo long > "$TREE/etc/$LONG"
+
+mkdir -p "$TREE/many"
+i=0
+while [ $i -lt 300 ]; do
+    echo "file $i" > "$TREE/many/f$i"
+    i=$((i + 1))
+done