lib-sieve: plugins: imap4flags: test-hasflag - Fix segfault occurring for command without arguments.

Providing an out-of-range index caused an assert panic.

Panic was:

Panic: file mail-index-transaction-update.c: line 19 (mail_index_transaction_lookup): assertion failed: (seq >= t->first_new_seq && seq <= t->last_new_seq)

Those are currently exclusively system tools and we cannot control whether those
have memory leaks.

Before, it would only terminate the test when it failed explicitly using the
test_fail command.

lib-sieve: Made sure local part of a mail address is encoded properly using quoted string when it is not a dot-atom.

It allows sending feedback report (RFC 5965) messages.

Mimics similar change in Dovecot.

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.

For some inapparent reason it was not included in the output tarball.

It now extracts bare text from HTML/XHMTL parts.
Other text/* parts are still returned as is. Any other unrecognized content types are skipped.

This adds the vnd.dovecot.default-mailbox and vnd.dovecot.username environment items.
It creates the 'env' variables namespace through which all environment items can be accessed directly.
This extension is yet to be documented properly.
More environment items are under consideration.

Also added testsuite items to test syntax.