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>

bcrypt/bcrypt.go

@@ -241,11 +241,11 @@ func (p *hashed) Hash() []byte {
 		n = 3
 	}
 	arr[n] = '$'
-	n += 1
+	n++
 	copy(arr[n:], []byte(fmt.Sprintf("%02d", p.cost)))
 	n += 2
 	arr[n] = '$'
-	n += 1
+	n++
 	copy(arr[n:], p.salt)
 	n += encodedSaltSize
 	copy(arr[n:], p.hash)