syscall: disallow slashes in file names on Plan 9

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/43480050

src/pkg/syscall/dir_plan9.go

@@ -11,6 +11,7 @@ import "errors"
 var (
 	ErrShortStat = errors.New("stat buffer too short")
 	ErrBadStat   = errors.New("malformed stat buffer")
+	ErrBadName   = errors.New("bad character in file name")
 )
 
 // A Qid represents a 9P server's unique identification for a file.
@@ -65,6 +66,12 @@ func (d *Dir) Marshal(b []byte) (n int, err error) {
 		return n, ErrShortStat
 	}
 
+	for _, c := range d.Name {
+		if c == '/' {
+			return n, ErrBadName
+		}
+	}
+
 	b = pbit16(b, uint16(n)-2)
 	b = pbit16(b, d.Type)
 	b = pbit32(b, d.Dev)