log file: if the path is not absolute make it relative to config dir

Also refuse to join invalid file name such as "."

Fixes #85

cmd/initprovider.go

@@ -1,6 +1,8 @@
 package cmd
 
 import (
+	"path/filepath"
+
 	"github.com/drakkan/sftpgo/config"
 	"github.com/drakkan/sftpgo/dataprovider"
 	"github.com/drakkan/sftpgo/logger"
@@ -29,6 +31,7 @@ Please take a look at the usage below to customize the options.`,
 		Run: func(cmd *cobra.Command, args []string) {
 			logger.DisableLogger()
 			logger.EnableConsoleLogger(zerolog.DebugLevel)
+			configDir = filepath.Clean(configDir)
 			config.LoadConfig(configDir, configFile)
 			providerConf := config.GetProviderConf()
 			logger.DebugToConsole("Initializing provider: %#v config file: %#v", providerConf.Driver, viper.ConfigFileUsed())