lib: fs: implement file cleanup using disconnect and detach events

lib/libfs/file.h

@@ -9,6 +9,9 @@
 #include <fs/inode.h>
 
 struct fs_file {
+	/* reference count includes the file descriptor, and any vm-objects
+	 * attached to the file */
+	unsigned int f_ref;
 	/* id of the open file, equal to the koid of the port being used to
 	 * access the file */
 	unsigned long f_id;