feat: Power-off fix for the Steam Machine; the kernel pin is optional

Kernels >= 7.2 keep the firmware's S4/S5 wake bit on GPIO pin 18, so the
Steam Machine starts again right after powering off. Steam Machine
support now builds a DKMS module (patches/steamify-fremont-poweroff.c)
that clears it before power-off, for every kernel. The kernel pin is no
longer ticked by default. Module sources and patches move to patches/,
embedded by the single-file build.

README.md

@@ -43,11 +43,13 @@ What Steamify can do on a Valve Steam Machine:
   TV's remote, and the TV turns on and off with the Steam Machine
   ([details](TECHNICAL.md#hdmi-cec)).
 - **Steam Machine support** - the front **LED bar** works, Steam's
-  **hardware settings** (fan, performance) work, and the power
-  button puts it to sleep like a console.
-  - **Pin the kernel to 7.1.6-1** - on by default: with newer CachyOS kernels
-    the Steam Machine reboots instead of shutting down. Untick it once CachyOS
-    fixes that. The kernel comes from its own
+  **hardware settings** (fan, performance) work, the power button puts it
+  to sleep like a console, and **shutting down really turns it off**, also
+  with the newest CachyOS kernel
+  ([details](TECHNICAL.md#power-off-fix-steam-machine)).
+  - **Pin the kernel to 7.1.6-1** - optional, off by default: stays on an
+    older kernel (HDMI refresh boost needs it). It used to be needed for
+    shutting down; the power-off fix above covers that now. The kernel comes from its own
     [release](https://github.com/theupriser/steamify-cachyos/releases/tag/kernel-7.1.6-1),
     checked against CachyOS's signature
     ([details](TECHNICAL.md#kernel-pin-steam-machine)).