Kernel: Make device generate their own names

Besides removing the monolithic DevFSDeviceInode::determine_name()
method, being able to determine a device's name inside the /dev
hierarchy outside of DevFS has its uses.

Kernel/Devices/RandomDevice.h

@@ -38,6 +38,7 @@ public:
 
     // ^Device
     virtual mode_t required_mode() const override { return 0666; }
+    virtual String device_name() const override { return "random"; }
 
 private:
     // ^CharacterDevice