add open and close functions, clean up ref_count=0 inodes and blocks

inc/fs.h

@@ -41,6 +41,11 @@ struct fs_directory_record {
 	unsigned int inode_no;
 };
 
+struct fs_file_description {
+	unsigned int inode;
+	unsigned int rw_offset;
+};
+
 char *fs_get_cwd(void);
 
 int fs_create(void *d);
@@ -50,5 +55,7 @@ int fs_mkfs(void *d);
 int fs_ls(void *d);
 int fs_la(void *d);
 int fs_rm(void *d);
+int fs_open(void *d);
+int fs_close(void *d);
 int fs_allow_write(void *d);
 int fs_prohibit_write(void *d);