Dovecot core doesn't have copyright notices in header files.

lib-sieve: Added support for having multiple instances of a single tag (type) within a single command.

Clang static analyzer is happy now.

Upon upload, the include extension is more lenient towards circular includes and missing scripts as required by RFC.
The script is now also verified upon SETACTIVE when it was not active before.
However, this new SETACTIVE behavior is not optimal for the situation where the active script was updated with
PUTSCRIPT and contains an nonexistent include (ignored during upload). This can still cause runtime errors, since
an already active script is not verified again during SETACTIVE.

lib-sieve: optimized compilation of tests that yield constant results (i.e. known at compile tme), such as true and false.
If the result of a test is known at compile time, it is optimized away. If an if-command depends on an entirely constant test, it is optimized away as well,
causing only the 'true' sub-block to be compiled.

Exported variable string argument into the Sieve engine itself as 'catenated string' (for similar use in other extensions like the testsuite).

Added the concept of persistent tags and implemented imapflags extension for bare keep and fileinto commands (intermittent commit, not working properly yet).