in dkim-signature header, allow FWS anywhere in "z=" (copied headers), and prevent panic in cli command "mox dkim verify" when a dkim-signature cannot be parsed

the BNF for "z=" does not show FWS is allowed (while it does in other places,
eg base64), but the text above the BNF explains it in words.

dkim/sig.go

@@ -213,7 +213,7 @@ func parseSignature(buf []byte, smtputf8 bool) (sig *Sig, verifySig []byte, err
 	ds := newSigWithDefaults()
 	seen := map[string]struct{}{}
 	p := parser{s: string(buf), smtputf8: smtputf8}
-	name := p.xhdrName()
+	name := p.xhdrName(false)
 	if !strings.EqualFold(name, "DKIM-Signature") {
 		xerrorf("%w", errSigHeader)
 	}