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/conf/dovecot.conf

@@ -149,7 +149,7 @@ service managesieve-login {
 plugin {
 	sieve_dir = ~/sieve
   sieve_plugins = sieve_extdata
-  sieve_extensions = +vnd.dovecot.extdata
+  sieve_extensions = +vnd.dovecot.extdata +spamtest +spamtestplus
 	sieve_before = /var/lib/dovecot/before.sieve
 	sieve_default = /var/lib/dovecot/default.sieve
 	sieve_after = /var/lib/dovecot/after.sieve
@@ -162,8 +162,13 @@ plugin {
   antispam_mail_spam = learn_spam
   antispam_mail_notspam = learn_ham
   antispam_mail_sendmail_args = -h;antispam:11334;-P;q1
-}
 
+  # extract spam score from
+  # X-Spam-Result: .... [<value> / <max_value] ...
+  sieve_spamtest_status_type = score
+  sieve_spamtest_status_header = X-Spamd-Result: .*\[(-?[[:digit:]]+\.[[:digit:]]+) .*\]
+  sieve_spamtest_max_header = X-Spamd-Result: .*\[.* ([[:digit:]]+\.[[:digit:]]+)\]
+}
 ###############
 # Filtering
 ###############