Kernel/Devices: Introduce the LoopDevice device

This device is a block device that allows a user to effectively treat an
Inode as a block device.

The static construction method is given an OpenFileDescription reference
but validates that:
- The description has a valid custody (so it's not some arbitrary file).
  Failing this requirement will yield EINVAL.
- The description custody points to an Inode which is a regular file, as
  we only support (seekable) regular files. Failing this requirement
  will yield ENOTSUP.

LoopDevice can be used to mount a regular file on the filesystem like
other supported types of (physical) block devices.

Kernel/Forward.h

@@ -19,6 +19,7 @@ class Coredump;
 class Credentials;
 class Custody;
 class Device;
+class DeviceControlDevice;
 class DiskCache;
 class DoubleBuffer;
 class File;
@@ -36,6 +37,7 @@ class Jail;
 class KBuffer;
 class KString;
 class LocalSocket;
+class LoopDevice;
 class Mutex;
 class MasterPTY;
 class Mount;