lib-sieve: redirect action: Made mail loop detection more robust by forcibly adding a Message-ID header if it is missing.

Struct mail_vfuncs has an additional method now.

This is needed for other extensions that manipulate flags while the imap4flags extension is not used directly.

This prevents a conflict between the message coming through a mailing list and a copy of that message recieved directly.
If both of these have the same message ID, one of these messages would be discarded (or worse: delivered locally) when redirected.

The caller will normally handle this and it will handle it better with more context information.
This also produced a spurious "script not found" error message when an entry but no attribute existed for an LDAP-based Sieve script.

lib-sieve: ldap storage: Clarified "script not found" errors a bit so that it is clear at which stage it occurrs.

lib-sieve: util: program-client-local: Changed link between parent and child process from a socketpair to a double pipe.

Turns out /dev/stdin, /dev/stdout, and /dev/fd/XX don't work on Linux if the underlying FD is a socket.
Since some programs may rely on those symbolic links to work, the local client is amended to use two pipes instead.
This will not help for programs invoked in the remote script service, since a socket is required for the connection.

Shutdown is now called only once.
If there is no used input side to the FD, it is now closed entirely.

Prevent mishaps with dup2(); e.g., when source and destination are identicial, which could happen.

The extensions listed in this setting do not need to be enabled explicitly using the Sieve require command.
This behavior voilates the standard, but is necessary for compatibility with some existing implementations of Sieve.
Do not use this setting unless you really need to!

Fixed a few printf format problems in the process.
Updated test suite to account for the new number width.

lib-sieve: code: Made jump offset code generation a bit more robust against integer overflow mishaps.

The actual version numbers live only on the release-branches from now on.

Mimics similar change in Dovecot.

Also made sure that any capability-derived state information is reset properly before a new capability reply is to be parsed.

lib-sieve: multiscript: Fixed bug in handling of (implicit) keep; final keep action was executed as though there was a failure.
Among other things, this caused the keep action to revert back to the initial message, causing editheader actions to be ignored.

Addresses LLVM scan-build report.

lib-sieve: storage: Local variable in sieve_storage_active_script_is_default() wasn't always properly initialized.
This was a LLVM scan-build report.

lib-sieve: imap4flags extension: Similar problem as previous commit, but relating to retrieval rather than manipulation.

Flags weren't always assigned properly, because the internal variable was not retrieved correctly.

lib-sieve: imap4flags extension: Forgot to check for empty flag/keyword in new flag checking function.