ssh: cosmetic cleanups

These are the cosmetic cleanups from the bits of code that I
rereviewed.

1) stringLength now takes a int; the length of the string. Too many
   callers were allocating with stringLength([]byte(s)) and
   stringLength only needs to call len().

2) agent.go now has sendAndReceive to remove logic that was
   duplicated.

3) We now reject negative DH values

4) We now reject empty packets rather than crashing.

R=dave, jonathan.mark.pittman
CC=golang-dev
https://golang.org/cl/6061052

ssh/client_auth.go

@@ -213,7 +213,7 @@ func (p *publickeyAuth) auth(session []byte, user string, t *transport, rand io.
 		}
 		// manually wrap the serialized signature in a string
 		s := serializeSignature(algoname, sign)
-		sig := make([]byte, stringLength(s))
+		sig := make([]byte, stringLength(len(s)))
 		marshalString(sig, s)
 		msg := publickeyAuthMsg{
 			User:     user,