Apply go fix to simplify using modern features

This patch contains the result of running `go fix ./...`, which applies
simplifications that make use of modern language features.

There are no logic or behaviour changes.

internal/config/config.go

@@ -27,8 +27,8 @@ var defaultConfig = &Config{
 
 	DataDir: "/var/lib/chasquid",
 
-	SuffixSeparators: proto.String("+"),
-	DropCharacters:   proto.String("."),
+	SuffixSeparators: new("+"),
+	DropCharacters:   new("."),
 
 	MailLogPath: "<syslog>",
 

internal/config/config_test.go

@@ -86,8 +86,8 @@ func TestFullConfig(t *testing.T) {
 
 		DataDir: "/var/lib/chasquid",
 
-		SuffixSeparators: proto.String(""),
-		DropCharacters:   proto.String(""),
+		SuffixSeparators: new(""),
+		DropCharacters:   new(""),
 
 		MailLogPath: "<syslog>",