crypto/hmac: Deprecate hmac.NewMD5, hmac.NewSHA1 and hmac.NewSHA256

Remove NewMD5, NewSHA1 and NewSHA256 in favor of using New and
explicitly importing the used hash-function. This way when using, for
example, HMAC with RIPEMD there's no md5, sha1 and sha256 linked in
through the hmac package.

A gofix rule is included, and applied to the standard library (3 files
altered).

This change is the result of a discussion at
https://golang.org/cl/5550043/ to pull the discussion about
deprecating these functions out of that issue.

R=golang-dev, agl
CC=golang-dev, r, rsc
https://golang.org/cl/5556058

cipher_suites.go

@@ -91,7 +91,7 @@ func macSHA1(version uint16, key []byte) macFunction {
 		copy(mac.key, key)
 		return mac
 	}
-	return tls10MAC{hmac.NewSHA1(key)}
+	return tls10MAC{hmac.New(sha1.New, key)}
 }
 
 type macFunction interface {