Look in more places for the dovecot binary

fix #510
Initialize dpath to make CI not fail
Special case for rejected messages.

see #516
Merge pull request #515 from carragom/1.1.x

Look in more places for the dovecot binary
Merge branch '1.1.x' of github.com:tonioo/modoboa into 1.1.x
Wrong regexp fixed.

see #516
Try to find the dovecot binary in more places.

see #510