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.

library/protocol/device-manager/device-manager-protocol.zig

@@ -12,7 +12,7 @@
 pub const version: u16 = 1;
 
 /// Which bus a `child_added` came from — stated by the reporting bus driver so
-/// the manager's /etc/devices.csv matcher knows how to read the report's identity
+/// the manager's /system/configuration/devices.csv matcher knows how to read the report's identity
 /// (a PCI class triple vs a USB class triple are the same 24 bits but different
 /// namespaces) and which `bus` column a rule must name to bind it. `unknown` is
 /// the zero default, so an un-upgraded reporter fails to match rather than
@@ -96,7 +96,7 @@ pub const ChildAdded = extern struct {
     /// for an unregistered leaf (a USB port before the descriptor track).
     device_id: u64 = no_device,
     /// The vendor id (PCI vendor / USB idVendor), or 0 when the bus has no such
-    /// concept (ACPI). Carried so the manager's /etc/devices.csv matcher can bind
+    /// concept (ACPI). Carried so the manager's /system/configuration/devices.csv matcher can bind
     /// on vendor — a level the bus-native `identity` (a class triple) cannot express.
     vendor: u16 = 0,
     /// The device id (PCI device / USB idProduct), or 0. The most specific numeric