Parallel to identical Dovecot change.

Some environment items in the base specification are not (yet) applicable to Dovecot and are therefore not supported.

Before, sievec and sieve-dump would switch to mail_uid:mail_gid and then fail to compile/dump a root script.

  - 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.

- 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.