build: the unix paths retire — configuration, logs, and volumes move into the danos tree

/etc/init.csv and /etc/devices.csv become /system/configuration/*.csv (the
repo's etc/ moves to system/configuration/, mirroring the runtime tree),
/var/log becomes /system/logs, and /mnt/usb becomes /volumes/usb. The
kernel VFS gains a carve-out so FAT may serve exactly /system/configuration
and /system/logs beneath the initrd-backed /system while /system and /test
themselves stay unshadowable; FAT's single /var mount splits into those two
rewritten mounts. The kvfs readdir check learns /system's third child and
the ramdisk spawn sweep skips the configuration tree.

Suite 106/106.

build/images.zig

@@ -83,7 +83,7 @@ pub fn addImageSteps(b: *std.Build, options: Options) std.Build.LazyPath {
     // holding the EFI stub, the kernel, and the whole /system tree of user
     // binaries at their FHS paths. QEMU presents this image as a USB mass-storage
     // device the guest boots from (see run-x86-64 and the test harness), and the
-    // danos fat driver mounts the same image at /mnt/usb.
+    // danos fat driver mounts the same image at /volumes/usb.
     const fat_image = addBootImage(b, options.kernel.getEmittedBin(), options.efi.getEmittedBin(), manifest_file, capsule_img, options.bundled);
     const fat_image_install = b.addInstallFile(fat_image, "danos-usb.img");
     b.getInstallStep().dependOn(&fat_image_install.step);