Kernel: Add File, a base class for anything that a FileDescriptor can wrap.

FileDescriptor is getting out of control, and the layering isn't quite right
so let's make a File class that everything can inherit from. Then we can
stop worrying about all kinds of specifics in FileDescriptor.

Kernel/File.cpp

@@ -0,0 +1,31 @@
+#include <Kernel/File.h>
+#include <Kernel/FileSystem/FileDescriptor.h>
+
+File::File()
+{
+}
+
+File::~File()
+{
+}
+
+KResultOr<Retained<FileDescriptor>> File::open(int options)
+{
+    UNUSED_PARAM(options);
+    return FileDescriptor::create(this);
+}
+
+void File::close()
+{
+}
+
+int File::ioctl(Process&, unsigned, unsigned)
+{
+    return -ENOTTY;
+}
+
+KResultOr<Region*> File::mmap(Process&, LinearAddress, size_t, size_t)
+{
+    return KResult(-ENODEV);
+}
+

Kernel/File.h

@@ -0,0 +1,36 @@
+#include <AK/Retainable.h>
+#include <AK/Types.h>
+#include <Kernel/FileSystem/FileDescriptor.h>
+
+class Process;
+
+class File : public Retainable<File> {
+public:
+    virtual ~File();
+
+    virtual KResultOr<Retained<FileDescriptor>> open(int options);
+    virtual void close();
+
+    virtual bool can_read(Process&) const = 0;
+    virtual bool can_write(Process&) const = 0;
+
+    virtual ssize_t read(Process&, byte*, ssize_t) = 0;
+    virtual ssize_t write(Process&, const byte*, ssize_t) = 0;
+    virtual int ioctl(Process&, unsigned request, unsigned arg);
+    virtual KResultOr<Region*> mmap(Process&, LinearAddress preferred_laddr, size_t offset, size_t size);
+
+    virtual String absolute_path() const = 0;
+
+    virtual const char* class_name() const = 0;
+
+    virtual bool is_seekable() const { return false; }
+
+    virtual bool is_device() const { return false; }
+    virtual bool is_tty() const { return false; }
+    virtual bool is_master_pty() const { return false; }
+    virtual bool is_block_device() const { return false; }
+    virtual bool is_character_device() const { return false; }
+
+protected:
+    File();
+};