smtpsrv: If the hook exits with code 20, it's a permanent failure

It is can be convenient for hooks to indicate that an error is
permanent; for example if the anti-virus found something.

This patch makes it so that if the hook exits with code 20, then it's
considered permanent. Otherwise it is considered transient, to help
prevent accidental errors cause final delivery issues.

hooks/post-data

@@ -5,6 +5,9 @@
 #
 #  - spamc (from Spamassassin) to filter spam.
 #  - clamdscan (from ClamAV) to filter virus.
+#
+# If it exits with code 20, it will be considered a permanent error.
+# Otherwise, temporary.
 
 set -e
 
@@ -19,7 +22,7 @@ cat > "$TF"
 if command -v spamc >/dev/null; then
         if ! SL=$(spamc -c - < "$TF") ; then
                 echo "spam detected"
-                exit 1
+                exit 20  # permanent
         fi
         echo "X-Spam-Score: $SL"
 fi
@@ -28,7 +31,7 @@ fi
 if command -v clamdscan >/dev/null; then
         if ! clamdscan --no-summary --infected - < "$TF" 1>&2 ; then
                 echo "virus detected"
-                exit 1
+                exit 20  # permanent
         fi
         echo "X-Virus-Scanned: pass"
 fi