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/asm.h

@@ -0,0 +1,21 @@
+//
+// assembler macros to create x86 segments
+//
+
+#define SEG_NULLASM                                             \
+        .word 0, 0;                                             \
+        .byte 0, 0, 0, 0
+
+// The 0xC0 means the limit is in 4096-byte units
+// and (for executable segments) 32-bit mode.
+#define SEG_ASM(type,base,lim)                                  \
+        .word (((lim) >> 12) & 0xffff), ((base) & 0xffff);      \
+        .byte (((base) >> 16) & 0xff), (0x90 | (type)),         \
+                (0xC0 | (((lim) >> 28) & 0xf)), (((base) >> 24) & 0xff)
+
+#define STA_X     0x8       // Executable segment
+#define STA_E     0x4       // Expand down (non-executable segments)
+#define STA_C     0x4       // Conforming code segment (executable only)
+#define STA_W     0x2       // Writeable (non-executable segments)
+#define STA_R     0x2       // Readable (executable segments)
+#define STA_A     0x1       // Accessed