Further improve boundary collision avoidance

The boundary generation system relies on a random generated boundary. E-mail boundaries can be nested in the same e-mail when e-mail contain subparts, and need to be strictliy different when inside the same message.

Although probablity of collision is low, we still rely on a non deterministic approach when it is realitively easy to make sure that boundaries are strictly different when generated inside the same message.

I am proposing this patch because I was indeed getting collisions every once in a while.

The proposed solution is to track a manually generated string that is to be appended to boundaries used during the generation of a single message + using SHA1 instead of MD5 to even further prevent collision of randomly generated boundaries.

Patch attached.