Kernel: Let inodes provide pre-open file descriptions

Some magical inodes, such as /proc/pid/fd/fileno, are going to want to open() to
a custom FileDescription, so add a hook for that.

Kernel/FileSystem/Inode.h

@@ -61,6 +61,8 @@ public:
     bool bind_socket(LocalSocket&);
     bool unbind_socket();
 
+    virtual FileDescription* preopen_fd() { return nullptr; };
+
     bool is_metadata_dirty() const { return m_metadata_dirty; }
 
     virtual int set_atime(time_t);