fixup! [sieve] add imap4flags awareness to sieve interpreter
fixup! [sieve] add parser support for imap4flags
fixup! [sieve] update sieve_fileinto() and sieve_keep() to use :flags parameter
fixup! [sieve] keep: ensure last flag list value wins (RFC 5232)
fixup! [sieve] fileinto: ensure last flag list value wins (RFC 5232)