Bring code closer to original Mail_mimeDecode
Bring code closer to original Mail_mimeDecode

This contains no functional changes and is mainly intended to
synchronize the code with the original Mail_mimeDecode. This makes it
easier to see what has been changed.

mta59066/Z-Push-contrib,Freeaqingme/Z-Push-contrib,fmbiete/Z-Push-contrib,RavenB/Z-Push-contrib