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.

include/device.h

@@ -0,0 +1,89 @@
+#pragma once
+
+// The device model: one list of everything attached, whatever bus found it.
+//
+// Before this there were two registries in kernel/drivers/registry.c -- one
+// array of PCI drivers, one of USB drivers -- and nothing that knew about both.
+// That is fine for binding drivers to hardware, which is all it was for, and
+// useless for the three things wanted next:
+//
+//   *Enumeration.* "What is in this machine, and what claimed it" had no
+//   answer that covered a USB disk and a PCI display at once. A device manager
+//   is exactly that question, so it needs exactly this list.
+//
+//   *Classes.* A compositor should ask for "the display" and not care whether
+//   that is the VBE framebuffer or virtio-gpu. Without a class, every consumer
+//   hardcodes a driver, and adding the second driver means editing the
+//   consumers -- which is how a framebuffer address ends up hardcoded in three
+//   places, as it once was here.
+//
+//   *A boundary for drivers that are not compiled in.* Drivers the user
+//   installs (see docs/drivers.md) have to attach somewhere that is not "an
+//   array in registry.c", because that array is fixed at link time.
+//
+// The shape is deliberately the one NeXTSTEP/macOS uses rather than Linux's:
+// a device is an object with a class and an ops vtable, and consumers talk to
+// the class. It is *not* a device tree -- there is no parent/child nesting
+// here, only a flat list with a bus tag, because nothing yet needs to walk a
+// hierarchy and a tree that nobody traverses is just a harder list.
+
+#define DEV_NAMELEN 24
+
+// Which bus found it. This is provenance, not capability -- what the device
+// *does* is its class.
+enum dev_bus {
+  DEVBUS_PLATFORM = 0,   // found by looking, not by enumerating: PS/2, VBE
+  DEVBUS_PCI,
+  DEVBUS_USB,
+  DEVBUS_NBUS
+};
+
+// What it is. A consumer asks for a class and gets whatever is bound.
+enum dev_class {
+  DEVCLASS_OTHER = 0,
+  DEVCLASS_DISPLAY,
+  DEVCLASS_INPUT,
+  DEVCLASS_STORAGE,
+  DEVCLASS_NET,
+  DEVCLASS_BUS,          // a controller that finds more devices (xHCI, hubs)
+  DEVCLASS_NCLASS
+};
+
+struct device {
+  char name[DEV_NAMELEN];      // "vga0", "xhci0" -- unique, human-readable
+  char desc[48];               // free text for the enumerator
+  enum dev_bus   bus;
+  enum dev_class class;
+
+  // Where it came from, for the enumerator to print. Zero when not applicable.
+  uint16 vendor, product;
+  uint8  b, s, f;              // PCI bus/slot/func
+
+  const char *driver;          // whatever claimed it, or 0 for unclaimed
+  void       *ops;             // class-specific vtable; see display.h
+  void       *priv;            // the driver's own state
+
+  int in_use;
+};
+
+#define NDEVICE 32
+
+// Register a device. Returns it, or 0 if the table is full. The name is made
+// unique by appending an index, so a caller passes "vga" and gets "vga0".
+struct device *device_add(const char *base, enum dev_bus bus,
+                          enum dev_class class, const char *driver);
+
+// Set the human-readable description. Bounded copy into the fixed field, so
+// callers do not each need to know how long it is.
+void device_set_desc(struct device *, const char *);
+
+// Walk. Returns 0 past the end; index is dense over registered devices.
+struct device *device_at(int i);
+int            device_count(void);
+
+// First device of a class with a bound driver, or 0. This is how a consumer
+// finds "the display" without naming a driver.
+struct device *device_find_class(enum dev_class class);
+
+const char *dev_bus_name(enum dev_bus);
+const char *dev_class_name(enum dev_class);