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/dev/acpi.h

@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#pragma once
+
+// ACPI, in two layers.
+//
+// kernel/arch/x86_64/acpi.c reads the static tables at boot, before anything
+// else can run: the MADT for the processors and the I/O APIC and its
+// interrupt source overrides, the FADT for the PM timer.
+//
+// kernel/acpi/ is everything that needs AML, which is the firmware's own code
+// and has to be interpreted: uACPI (ports/uacpi) runs it, and aml.c uses it
+// for PCI interrupt routing, the power button, power-off and reset. Every
+// function below that depends on it does nothing, and says so by returning
+// -1, on a machine whose AML could not be loaded.
+
+// The RSDP's physical address, or 0 if there is none.
+uint64 acpi_rsdp_phys(void);
+
+// The GSI an ISA IRQ is wired to, after the MADT's interrupt source
+// overrides, and its trigger mode (an IRQ_* from irq.h) in *mode. `dflt` is
+// the mode when the override says "conforms to the bus" or there is none:
+// edge/high for an ISA device, level/low for the SCI.
+int    acpi_isa_gsi(int isa, int dflt, int *mode);
+
+// Load and initialize the namespace. After kinit, before pci_init.
+void   acpi_aml_init(void);
+// The worker thread that runs AML on behalf of interrupts. After sinit.
+void   acpi_thread_start(void);
+// Route every PCI device's INTx through _PRT. From pci_init, before binding.
+void   acpi_pci_route(void);
+
+int    acpi_poweroff(void);     // S5; returns only on failure
+int    acpi_reboot(void);       // the FADT reset register; ditto