chasquid-util: switch --configdir into --config_dir and deprecate --configdir

There's an inconsistency between chasquid (which uses `--config_dir`) and
chasquid-util (which uses `--configdir`).

That is prone to cause confusion, so this patch renames chasquid-util's
flag, leaving the old one as deprecated with a warning message.

Closes https://github.com/albertito/chasquid/pull/60.

Amended-by: Alberto Bertogli <albertito@blitiri.com.ar>
  Added test case for the deprecated option, adjusted commit message.

cmd/chasquid-util/chasquid-util.go

@@ -48,8 +48,8 @@ Usage:
     private key.
 
 Options:
-  -C=<path>, --configdir=<path>  Configuration directory
-  -v                             Verbose mode
+  -C=<path>, --config_dir=<path>  Configuration directory
+  -v                              Verbose mode
 `
 
 // Command-line arguments.
@@ -74,12 +74,16 @@ func main() {
 	}
 
 	// Load globals.
-	if d, ok := args["--configdir"]; ok {
+	if d, ok := args["--config_dir"]; ok {
 		configDir = d
 	}
 	if d, ok := args["-C"]; ok {
 		configDir = d
 	}
+	if d, ok := args["--configdir"]; ok {
+		configDir = d
+		Warnf("Option --configdir is deprecated, use --config_dir instead")
+	}
 
 	commands := map[string]func(){
 		"user-add":          userAdd,
@@ -114,6 +118,11 @@ func Fatalf(s string, arg ...interface{}) {
 	os.Exit(1)
 }
 
+// Warnf prints the given message to stderr, but does not exit the program.
+func Warnf(s string, arg ...interface{}) {
+	fmt.Fprintf(os.Stderr, s+"\n", arg...)
+}
+
 func userDBForDomain(domain string) string {
 	if domain == "" {
 		domain = args["$2"]