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>

openpgp/keys.go

@@ -325,9 +325,8 @@ func ReadEntity(packets *packet.Reader) (*Entity, error) {
 		if e.PrivateKey, ok = p.(*packet.PrivateKey); !ok {
 			packets.Unread(p)
 			return nil, errors.StructuralError("first packet was not a public/private key")
-		} else {
-			e.PrimaryKey = &e.PrivateKey.PublicKey
 		}
+		e.PrimaryKey = &e.PrivateKey.PublicKey
 	}
 
 	if !e.PrimaryKey.PubKeyAlgo.CanSign() {