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/cipher.go

@@ -35,10 +35,10 @@ func newRC4(key, iv []byte) (cipher.Stream, error) {
 }
 
 type cipherMode struct {
-	keySize  int
-	ivSize   int
-	skip     int
-	createFn func(key, iv []byte) (cipher.Stream, error)
+	keySize    int
+	ivSize     int
+	skip       int
+	createFunc func(key, iv []byte) (cipher.Stream, error)
 }
 
 func (c *cipherMode) createCipher(key, iv []byte) (cipher.Stream, error) {
@@ -49,7 +49,7 @@ func (c *cipherMode) createCipher(key, iv []byte) (cipher.Stream, error) {
 		panic("ssh: iv too small for cipher")
 	}
 
-	stream, err := c.createFn(key[:c.keySize], iv[:c.ivSize])
+	stream, err := c.createFunc(key[:c.keySize], iv[:c.ivSize])
 	if err != nil {
 		return nil, err
 	}