include: one folder per subsystem, and includes that name it

Sixty-five headers sat side by side in include/, so `#include "irq.h"`
said nothing about where irq.h was or what it belonged to, and errno.h,
stat.h and fcntl.h at the top shadowed the system's for any host tool
built with -I./include. They are in folders now -- arch/ core/ fs/
dev/ gfx/ net/ uapi/ (and ldls/ for the Linux driver layer) -- and
every include names the folder, as Linux's do: "core/proc.h".

Done mechanically and checked the hard way: the kernel built from this
tree disassembles identically to the one built from the tree before
it. The full image boots, and userland is the same set of programs.
Paths in docs and comments follow; docs/guides/layout.md has a table
of what is where, and tools/fix-includes.py rewrites old-style
includes from the folders in the tree, for code written before this.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

include/arch/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