Fix evaluating susemailforwardaddress atribute bnc#808641

In this patch following bugs was fixed:
- bnc#808641 - yast2-mail advanced configuration can create mail loops
- bnc#759050 - autoyast mail configuration (postfix) not possible
- bnc#800788 - ag_postfix_mastercf process hangs the system, after which it is killed by kernel, when saving mail server settings with Yast "Mail Server" module