Clang static analyzer is happy now.

  - Built generic interface for alternative script sources.
  - Implemented dict script location.
  NOTE: ManageSieve will not work with this yet, nor will sieve_before/sieve_after.

- Added sieve_global_extensions setting.

Configuration is now only read once at extension initialization.

- Administrators now get properly notified about uncompiled global scripts and the inability of the sieve plugin to store global binaries.
- Improved binary load/save error handling and fixed a few smal bugs.
- Simplified ManageSieve error handling.

Testsuite: added tests on the envelope of outgoing messages produced by redirect, enotify:mailto, reject and vacation.

Major rework of envelope address handling: a normalized version of the envelope addresses is maintained in the message context and message context is now also available during action execution.