Refactor parsing of -listen string out into separate config function

This makes the "for each listen address" loop in main() look even cleaner.

config_test.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestSplitProto(t *testing.T) {
+	var tests = []struct {
+		input      string
+		proto      string
+		addr       string
+	}{
+		{
+			input:      "localhost",
+			proto:      "",
+			addr:       "localhost",
+		},
+		{
+			input:      "tls://my.local.domain",
+			proto:      "tls",
+			addr:       "my.local.domain",
+		},
+		{
+			input:      "starttls://my.local.domain",
+			proto:      "starttls",
+			addr:       "my.local.domain",
+		},
+	}
+
+	for i, test := range tests {
+		testName := test.input
+		t.Run(testName, func(t *testing.T) {
+			pa := splitProto(test.input)
+			if pa.protocol != test.proto {
+				t.Errorf("Testcase %d: Incorrect proto: expected %v, got %v",
+					i, test.proto, pa.protocol)
+			}
+			if pa.address != test.addr {
+				t.Errorf("Testcase %d: Incorrect addr: expected %v, got %v",
+					i, test.addr, pa.address)
+			}
+		})
+	}
+}