- Tweak the scheduler API. The scheduler backend can now return a mix
  of different envelope, instead of batches of a single type.
- Interlace envelopes from all queue types in the default scheduler.
- Allow to use an external proc as scheduler backend.
- Implement a python glue for the scheduler API.
fix warnings
Merge branch 'master' of ssh://ssh.poolp.org/git/opensmtpd