tlog: disable color codes when switching to syslog

When gocryptfs was started on a terminal and later
daemonized, the color codes stayed active in the syslog
output.

The codes are not visible in "journalctl -f", which is why
I have not noticed it yet, but they do show up in normal
syslog as the usual "#033[33m" crap.

internal/tlog/log.go

@@ -142,6 +142,9 @@ func (l *toggledLogger) SwitchToSyslog(p syslog.Priority) {
 		Warn.Printf("SwitchToSyslog: %v", err)
 	} else {
 		l.Logger.SetOutput(w)
+		// Disable colors
+		l.prefix = ""
+		l.postfix = ""
 	}
 }