Reopen logs on SIGHUP

This makes it possible to manage chasquid logs using logrotate.

Amended-by: Alberto Bertogli <albertito@blitiri.com.ar>
  Added tests, minor style and comment changes.

internal/maillog/maillog.go

@@ -81,6 +81,11 @@ func (l *Logger) printf(format string, args ...interface{}) {
 	}
 }
 
+// Reopen the underlying logger.
+func (l *Logger) Reopen() error {
+	return l.inner.Reopen()
+}
+
 // Listening logs that the daemon is listening on the given address.
 func (l *Logger) Listening(a string) {
 	l.printf("daemon listening on %s\n", a)