Introduce go-smtp

[mabels]

Hello,

the current mhale/smtpd has leaks in file descriptors and is not actively maintained. That's why i switched to go-smtp.

This switch wasn't big but i tried to build tests for the all uncovered sections and the leaked file descriptors. That's why this change is a kind of big. Sorry for that but in decision of compact changes or better testablity i choose testablity.

Meno