Integer type policy

Integers are for numbers, enabling arithmetic like subtractions and for
loops without getting shot in the foot. Unsigneds are for bitfields.

- "int" for numbers that will always be laughably smaller than four
  billion, and where we don't care about the serialization format.

- "int32" for numbers that will always be laughably smaller than four
  billion, and will be serialized to four bytes.

- "int64" for numbers that may approach four billion or will be
  serialized to eight bytes.

- "uint32" and "uint64" for bitfields, depending on required number of
  bits and serialization format. Likewise "uint8" and "uint16", although
  rare in this project since they don't exist in XDR.

- "int8", "int16" and plain "uint" are almost never useful.

internal/model/model_test.go

@@ -110,6 +110,33 @@ func TestRequest(t *testing.T) {
 	if bs != nil {
 		t.Errorf("Unexpected non nil data on insecure file read: %q", string(bs))
 	}
+
+	// Larger block than available
+	bs, err = m.Request(device1, "default", "foo", 0, 42)
+	if err == nil {
+		t.Error("Unexpected nil error on insecure file read")
+	}
+	if bs != nil {
+		t.Errorf("Unexpected non nil data on insecure file read: %q", string(bs))
+	}
+
+	// Negative offset
+	bs, err = m.Request(device1, "default", "foo", -4, 6)
+	if err == nil {
+		t.Error("Unexpected nil error on insecure file read")
+	}
+	if bs != nil {
+		t.Errorf("Unexpected non nil data on insecure file read: %q", string(bs))
+	}
+
+	// Negative size
+	bs, err = m.Request(device1, "default", "foo", 4, -4)
+	if err == nil {
+		t.Error("Unexpected nil error on insecure file read")
+	}
+	if bs != nil {
+		t.Errorf("Unexpected non nil data on insecure file read: %q", string(bs))
+	}
 }
 
 func genFiles(n int) []protocol.FileInfo {