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/irq.h

@@ -0,0 +1,43 @@
+#pragma once
+
+// Hardware interrupt registration.
+//
+// Until this existed, trap() was a switch statement listing every interrupt
+// the kernel knew about, and adding a device meant editing the trap handler.
+// That is fine while the devices are the four the PC has always had at fixed
+// pins; it stops being fine the moment a device's IRQ is whatever the BIOS
+// decided to put in its PCI config space this boot. A driver cannot write
+// `case T_IRQ0 + IRQ_XHCI:` because there is no such constant -- it only
+// learns its line at probe time.
+//
+// So: a line has a chain of handlers, a driver adds itself to one at probe
+// time, and enabling the line is a consequence of registering rather than a
+// separate call somebody can forget.
+//
+// Chains matter more than they look. PCI interrupt pins are *shared*: four
+// INTx lines are routed to a handful of IOAPIC inputs, so two unrelated
+// devices routinely land on the same one. A handler must therefore check
+// whether its own device actually raised the interrupt and do nothing if it
+// did not.
+
+#define NIRQ    24      // IOAPIC redirection entries on a PC
+#define NCHAIN   4      // handlers that may share one line
+
+typedef void (*irq_handler_t)(void *arg);
+
+// Returns 0, or -1 if the line is out of range, reserved, or full.
+// Registering enables the line; the last unregister masks it again.
+//
+// `level_low` selects PCI's signalling (level-triggered, active low) rather
+// than ISA's (edge-triggered, active high). Getting this wrong does not fail
+// cleanly: an edge-configured level interrupt fires once and then wedges,
+// because the device keeps the line asserted and no further edge ever comes.
+int  irq_register(int irq, irq_handler_t fn, void *arg, const char *name,
+                  int level_low);
+void irq_unregister(int irq, irq_handler_t fn, void *arg);
+
+// Called from trap() with the IOAPIC pin number, not the vector.
+void irq_dispatch(int irq);
+
+void irq_init(void);
+int  irq_info(char *buf, int cap);   // /proc/interrupts