hooks: Use chasquid-rspamd if available

The chasquid-rspamd utility (https://github.com/Thor77/chasquid-rspamd)
provides a better integration with rspamd, by taking envelope and
connection information from the environment variables, and communicating
with rspamd using its protocol.

So if it is available, use it instead of rspamc in the default hook.

etc/chasquid/hooks/post-data

@@ -5,7 +5,7 @@
 #
 #  - greylist (from greylistd) to do greylisting.
 #  - spamc (from Spamassassin) to filter spam.
-#  - rspamc (from rspamd) to filter spam.
+#  - rspamc (from rspamd) or chasquid-rspamd to filter spam.
 #  - clamdscan (from ClamAV) to filter virus.
 #  - dkimsign (from driusan/dkim) to do DKIM signing.
 #
@@ -47,7 +47,13 @@ if command -v spamc >/dev/null; then
 fi
 
 
-if command -v rspamc >/dev/null; then
+# Spam filter through rspamd.
+#
+# Use chasquid-rspamd (from https://github.com/Thor77/chasquid-rspamd) if
+# available, otherwise fall back to rspamc.
+if command -v chasquid-rspamd >/dev/null; then
+	chasquid-rspamd < "$TF" 2>/dev/null
+elif command -v rspamc >/dev/null; then
 	# Note the actions emitted by rspamc come from the thresholds
 	# configured in /etc/rspamd/actions.conf.
 	# The ones handled here are common defaults, but they might require