dovecot: use rspamd X-Spamd-Result percentage to evaluate spam

- configures dovecot to use the spamtest sieve plugins
- configures sieve to read the score from X-Spamd-Result: headers
- before.sieve applies the ${spam_threshold} to the spamtest percentage
- freeposte.db stores a percentage for ${spam_threshold}
- migrate freeposte.db spam_threshold from X/15 to percentages

the filter investigates the overall ratio of the `rspamd` header
`X-Spamd-Result` that looks something like this:

X-Spamd-Result: default: True [12.36 / 15.00]
 RBL_SPAMHAUS_XBL(4.00)[]
 BAYES_SPAM(3.06)[92.67%]
 RBL_SPAMHAUS_XBL_ANY(4.00)[]
 ONCE_RECEIVED_STRICT(4.00)[]
 HFILTER_HELO_BAREIP(3.00)[]
 RBL_SORBS_DUL(2.00)[]
 HFILTER_HOSTNAME_UNKNOWN(2.50)[]
 RBL_SPAMHAUS_PBL(2.00)[]
 RBL_SORBS_RECENT(1.50)[]
 MIME_UNKNOWN(0.10)[application/x-rar-compressed]
 RDNS_NONE(1.00)[]
 RBL_SORBS(0.00)[]
 R_SPF_NEUTRAL(0.00)[?all]
 ONCE_RECEIVED(0.10)[]
 RBL_SEM(1.00)[]
 MIME_HTML_ONLY(0.20)[]
 RBL_UCEPROTECT_LEVEL1(1.00)[]
 MIME_GOOD(-0.10)[multipart/mixed]

the sieve `spamtest :percent :value` in this case would be
   100*12.36/15 = 82.4%

dovecot/sieve/before.sieve

@@ -8,16 +8,14 @@ require "regex";
 require "relational";
 require "comparator-i;ascii-numeric";
 require "vnd.dovecot.extdata";
+require "spamtestplus";
 
 if allof (string :is "${extdata.spam_enabled}" "1",
-          not header :matches "X-Spam-Status" "* score=-*",
-          header :matches "X-Spam-Status" "* score=*")
+          spamtest :percent :value "gt" :comparator "i;ascii-numeric"  "${extdata.spam_threshold}")
 {
-  if string :value "ge" :comparator "i;ascii-numeric" "${2}" "${extdata.spam_threshold}" {
-    setflag "\\seen";
-    fileinto :create "Junk";
-    stop;
-  }
+  setflag "\\seen";
+  fileinto :create "Junk";
+  stop;
 }
 
 if string :is "${extdata.reply_enabled}" "1" {