exp/ssh: fix unmarshal test

Ensure that empty NameLists always return
a zero length []string, not nil.

In practice NameLists are only used in a few
message types and always consumed by a for
range function so the difference between nil
and []string{} is not significant.

Also, add exp/ssh to pkg/Makefile as suggested
by rsc.

R=rsc, agl
CC=golang-dev
https://golang.org/cl/5400042

src/pkg/exp/ssh/messages.go

@@ -392,7 +392,10 @@ func parseString(in []byte) (out, rest []byte, ok bool) {
 	return
 }
 
-var comma = []byte{','}
+var (
+	comma         = []byte{','}
+	emptyNameList = []string{}
+)
 
 func parseNameList(in []byte) (out []string, rest []byte, ok bool) {
 	contents, rest, ok := parseString(in)
@@ -400,6 +403,7 @@ func parseNameList(in []byte) (out []string, rest []byte, ok bool) {
 		return
 	}
 	if len(contents) == 0 {
+		out = emptyNameList
 		return
 	}
 	parts := bytes.Split(contents, comma)