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>

terminal.go

@@ -617,7 +617,7 @@ func writeWithCRLF(w io.Writer, buf []byte) (n int, err error) {
 			if _, err = w.Write(crlf); err != nil {
 				return n, err
 			}
-			n += 1
+			n++
 			buf = buf[1:]
 		}
 	}