[edit] Add line-editing library, libedit

Now used by the 6s shell to enable backspace, return, etc. No cursor
movement yet or anything complex.

src/user/6s/6s.module

@@ -2,7 +2,7 @@
 
 module("6s",
     targets = [ "user" ],
-    deps = [ "libc" ],
+    deps = [ "libc", "edit" ],
     description = "j6 shell",
     sources = [
         "main.cpp",