all: fix errors reported by vet, golint

None are "wrong" per se, but there are a lot of good suggestions and
in one case a docstring that was not present in godoc due to the
presence of an extra newline.

Changed "Id" in struct properties to "ID" in some non-exported
structs. Removed a trailing period from some error messages; I believe
the exact contents of error strings are not covered by the Go
compatibility promise.

Change-Id: I7c620582dc247396f72c52d38c909ccc0ec87b83
Reviewed-on: https://go-review.googlesource.com/80145
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

ssh/common.go

@@ -242,7 +242,7 @@ func (c *Config) SetDefaults() {
 
 // buildDataSignedForAuth returns the data that is signed in order to prove
 // possession of a private key. See RFC 4252, section 7.
-func buildDataSignedForAuth(sessionId []byte, req userAuthRequestMsg, algo, pubKey []byte) []byte {
+func buildDataSignedForAuth(sessionID []byte, req userAuthRequestMsg, algo, pubKey []byte) []byte {
 	data := struct {
 		Session []byte
 		Type    byte
@@ -253,7 +253,7 @@ func buildDataSignedForAuth(sessionId []byte, req userAuthRequestMsg, algo, pubK
 		Algo    []byte
 		PubKey  []byte
 	}{
-		sessionId,
+		sessionID,
 		msgUserAuthRequest,
 		req.User,
 		req.Service,