[Dovecot] dovecot-sieve vacation changes

[pod]

I'd like to put forward the following patch for dovecot-sieve.

Essentially this just merges in some changes made in CMU sieve 2.3.8 and
as such brings the behaviour more in line with RFC 3834 "Recommendations
for Automatic Responses to Electronic Mail":

 - fixes erroneous sender "-request" substring match

 - checks for existence of a selection of list-* headers

 - expands "named-as-recipient" header checks to "Resent-To", "Resent-CC"
   and "Resent-BCC" fields.  Note that the appended patch also includes a
   fix for some missing parentheses in the "resent-to" test which resulted
   in some extra work being done but was otherwise harmless.  The fix has
   been sent to <cyrus-bugs+ at andrew.cmu.edu>

 - prefixes subject in response with "Auto: " rather than (non-expanding)
   "Re: "

I also, mostly for kicks, attach two sieve-like scripts that attempt to
encapsulate the when-not-to-respond logic as implemented (if the patch
were to be applied) and as advised by RFC 3834 respectively.  I wrote
these as an exercise to help me gain a better understanding of sieve and
the CMU implementation.  I'm not sure they are otherwise useful.
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