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>

xtea/cipher.go

@@ -69,7 +69,7 @@ func initCipher(c *Cipher, key []byte) {
 
 	// Precalculate the table
 	const delta = 0x9E3779B9
-	var sum uint32 = 0
+	var sum uint32
 
 	// Two rounds of XTEA applied per loop
 	for i := 0; i < numRounds; {