Minor code aesthetic improvements, based on vet+fmt+lint

This patch is the result of running go vet, go fmt -s and the linter,
and fixing some of the things they noted/suggested.

There shouldn't be any significant logic changes, it's mostly
readability improvements.

chasquid.go

@@ -235,6 +235,7 @@ func mustReadDir(path string) []os.FileInfo {
 // We keep them distinct, as policies can differ between them.
 type SocketMode string
 
+// Valid socket modes.
 const (
 	ModeSMTP       SocketMode = "SMTP"
 	ModeSubmission SocketMode = "Submission"