[6s] Allow 6s to know about filesystems

Added a j6_proto_vfs_tag/_get_tag pair of messages to the VFS protocol
to allow filesystems to label themselves, and gave 6s the concept of
current fs and current working directory.

src/user/6s/6s.module

@@ -7,4 +7,5 @@ module("6s",
     sources = [
         "commands.cpp",
         "main.cpp",
+        "shell.cpp",
     ])