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.

.gitignore

@@ -0,0 +1,24 @@
+# Build output. Everything here is reproducible from `make`.
+build/
+schunix.img
+schunix.iso
+*.o
+*.d
+*.asm
+*.sym
+
+# The staged root filesystem. Assembled by the build from cmd/, lib/ and
+# ports/ -- tracking it would mean committing a second copy of every binary
+# on every build.
+sysroot/
+
+# Upstream tarballs, fetched not authored: musl, toybox, mksh, dash, the DOOM
+# WADs and usb.ids. 61 MB of other people's release artefacts, all of which
+# the build downloads again if they are missing. The patches that matter are
+# under ports/*/patches/ and those *are* tracked.
+dist/
+
+# Test and screenshot output.
+*.log
+*.ppm
+*.png.tmp