mkfs: if neither --dotdot or --dot options given, create .. link to current dir
accept -1 for inum in add_direntry to mean "use previous i-num" for links
proto: use inum -1 to make "list" a link to "nls"
mkfs: if neither --dotdot or --dot options given, create .. link to current dir
accept -1 for inum in add_direntry to mean "use previous i-num" for links
proto: use inum -1 to make "list" a link to "nls"