Refactor client into separate files

Add a main loop, move I/O code into io.go, move configuration into config.go.
Use shared.Decode24BitLength instead of manually decoding block lengths.
Fix response block decoding and checksum validation.
Add constants for commonly used byte values and use these in the signer and
the client.

client/config.go

@@ -0,0 +1,84 @@
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"time"
+
+	"github.com/goburrow/serial"
+	"github.com/sirupsen/logrus"
+	"gopkg.in/yaml.v2"
+)
+
+type SerialConfig struct {
+	Address  string `yaml:"address"`
+	BaudRate int    `yaml:"baudrate"`
+	DataBits int    `yaml:"databits"`
+	StopBits int    `yaml:"stopbits"`
+	Parity   string `yaml:"parity"`
+}
+
+type ClientConfig struct {
+	Serial        SerialConfig `yaml:"serial_config"`
+	Paranoid      bool         `yaml:"paranoid"`
+	Debug         bool         `yaml:"debug"`
+	GNUPGBinary   string       `yaml:"gnupg_bin"`
+	OpenSSLBinary string       `yaml:"openssl_bin"`
+	MySQLDSN      string       `yaml:"mysql_dsn"`
+}
+
+var defaultConfig = ClientConfig{
+	Serial: SerialConfig{
+		Address:  "/dev/ttyUSB0",
+		BaudRate: 115200,
+		DataBits: 8,
+		StopBits: 1,
+		Parity:   "N",
+	},
+	Paranoid:      false,
+	Debug:         false,
+	OpenSSLBinary: "/usr/bin/openssl",
+	GNUPGBinary:   "/usr/bin/gpg",
+	MySQLDSN:      "<username>:<password>@/database?parseTime=true",
+}
+
+func generateExampleConfig(configFile string) (config *ClientConfig, err error) {
+	config = &defaultConfig
+	configBytes, err := yaml.Marshal(config)
+	if err != nil {
+		logrus.Errorf("could not generate configuration data")
+		return
+	}
+
+	logrus.Infof("example data for %s:\n\n---\n%s\n", configFile, configBytes)
+	return
+}
+
+func readConfig(configFile string) (config *ClientConfig, err error) {
+	source, err := ioutil.ReadFile(configFile)
+	if err != nil {
+		logrus.Errorf("opening configuration file failed: %v", err)
+		if exampleConfig, err := generateExampleConfig(configFile); err != nil {
+			return nil, err
+		} else {
+			logrus.Info("starting with default config")
+			return exampleConfig, nil
+		}
+	}
+	if err := yaml.Unmarshal(source, &config); err != nil {
+		return nil, fmt.Errorf("loading configuration file failed: %v", err)
+	}
+
+	return config, nil
+}
+
+func fillSerialConfig(clientConfig *ClientConfig) *serial.Config {
+	return &serial.Config{
+		Address:  clientConfig.Serial.Address,
+		BaudRate: clientConfig.Serial.BaudRate,
+		DataBits: clientConfig.Serial.DataBits,
+		StopBits: clientConfig.Serial.StopBits,
+		Parity:   clientConfig.Serial.Parity,
+		Timeout:  30 * time.Second,
+	}
+}