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/memlayout.h

@@ -0,0 +1,49 @@
+#pragma once
+
+// Memory layout
+
+#define EXTMEM  0x100000            // Start of extended memory
+// Top of usable physical memory. A *variable*, read from the firmware's E820
+// map at boot -- see kernel/arch/x86_64/e820.c. It used to be the constant
+// below, which was not a limit on what the kernel used but an assertion that
+// exactly 224 MB existed: less and the allocator walked off the end of RAM,
+// more and it was invisible.
+//
+// The macro spelling is kept so that the twenty-odd call sites read the same as
+// they always did. It must not be used before e820_init(), which runs first
+// thing in main() for that reason.
+#ifndef __ASSEMBLER__
+extern uint64 phystop;
+#define PHYSTOP phystop
+#endif
+
+// What to assume when the firmware gives us nothing usable: the old constant,
+// so that path behaves exactly as the kernel always did.
+#define PHYSTOP_FALLBACK 0xE000000ULL
+
+// kvmalloc() maps physical 0-4 GB as four 1 GB pages and marks the top one
+// uncached, because that is where PCI BARs and the framebuffer sit. 3 GB is
+// therefore the most that can be treated as RAM.
+#define PHYSTOP_MAX      0xC0000000ULL
+
+// Below this the kernel image, its page tables and the DMA arena do not fit.
+#define PHYSTOP_MIN      (24ULL * 1024 * 1024)
+
+// Key addresses for address space layout
+#define KERNBASE 0xFFFF800000000000 // First kernel virtual address
+#define KERNLINK (KERNBASE+EXTMEM)  // Address where kernel is linked
+
+#ifndef __ASSEMBLER__
+static inline addr_t v2p(void *a) {
+  return ((addr_t)(a)) - ((addr_t)KERNBASE);
+}
+static inline void *p2v(addr_t a) {
+  return (void *)((a) + ((addr_t)KERNBASE));
+}
+#endif
+
+#define V2P(a) (((addr_t) (a)) - KERNBASE)
+#define P2V(a) (((void *) (a)) + KERNBASE)
+
+#define V2P_WO(x) ((x) - KERNBASE)    // same as V2P, but without casts
+#define P2V_WO(x) ((x) + KERNBASE)    // same as P2V, but without casts