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/agent/server.go

@@ -148,7 +148,7 @@ func (s *server) processRequest(data []byte) (interface{}, error) {
 		}
 		return rep, nil
 
-	case agentAddIdConstrained, agentAddIdentity:
+	case agentAddIDConstrained, agentAddIdentity:
 		return nil, s.insertIdentity(data)
 	}