lib-sieve: made sure that modified messages are not stored in the source mailbox when it was opened read-only.
This is currently only relevant for the sieve-filter tool to avoid duplicating messages in the
source folder. This can happen when the messages are modified (e.g. by the editheader extension) and the
source folder is opened read-only, thus preventing deletion of the original message.

lib-sieve: store action: copy flags and keywords from input mail, also when imap4flags extension is not active.
This is currently only relevant for sieve-filter, which now properly preserves flags with this change.

- Added sieve_global_extensions setting.

- Now compiles regularly without --with-unfinished-features
- Still experimental though, so be careful.
- Changed command structure a bit, removing the useless -M option.

- Updated documentation.
- Made editheader extension disabled by default

This was broken by previous change. It now properly uses the final version of the message
instead of the original.

It did not use the original message in this case.

lib-sieve: editheader extension: fixed handling of pre-folded header values for added header fields.

lib-sieve: updated rfc2822 header field body verification to exclude non-printing characters (RFC5322).

This change matches identical changes for Dovecot's imap-login and pop3-login.