Spamassassin tuning, ignore big files and add feature to reject mails wi...

...th high spam-score directly.

In some old configs, I found some improvements concerning the handling with spam mail:
- To save resources, only mails larger than 100 KB are scanned because typical spam is only about a few KB (per default on).
- The second option is to reject mails with spam scores higher than 15 directly. The interest is to avoid the handling of mails with a high spamassassin score. This concerns especially addresses which are forwarded to other  servers (e.g. googlemail). These forwarded messages are often rejected by the other server, then our server creates bounce-massages (backscatter) and can obtain a bad reputation. It is off by default. You have to verify first, that spamassassin is properly working, 15 is a more or less random value.