write base64 message parts with 76 data bytes on a line instead of 78

As required by RFC 2045 (MIME). The 78 byte lines work in practice, except that
SpamAssassin has rules that give messages with 78-byte lines spam points.

Mentioned by kjetilho on irc.

message/qp.go

@@ -11,7 +11,9 @@ import (
 func NeedsQuotedPrintable(text string) bool {
 	// ../rfc/2045:1025
 	for _, line := range strings.Split(text, "\r\n") {
-		if len(line) > 78 || strings.Contains(line, "\r") || strings.Contains(line, "\n") {
+		// 78 should be fine too, qp itself has a requirement of 76 bytes on a line, but
+		// using qp for anything longer than 76 is safer.
+		if len(line) > 76 || strings.Contains(line, "\r") || strings.Contains(line, "\n") {
 			return true
 		}
 	}