Guard: watch logs, ban misbehaving addresses in Housecarl

Files and the journal, patterns with one <IP>, bounties within a window,
bans in a Housecarl ban list with repeat offenders' bans growing; presets
for sshd, jarl, housecarl, steward, bind-xfr, postfix-sasl, dovecot,
nginx-auth and housecarl-drops; guard run/check/test/status/bans/ban/unban;
a Debian package with a sandboxed unit; an end-to-end test with Housecarl.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/debian/postrm

@@ -0,0 +1,17 @@
+#!/bin/sh
+# Guard: runs after the package's files are removed.
+set -e
+
+case "$1" in
+remove|purge)
+	if [ -d /run/systemd/system ]; then
+		systemctl daemon-reload || true
+	fi
+	;;
+esac
+# The bans Guard made stay in Housecarl until they end.
+if [ "$1" = purge ]; then
+	echo "guard: kept /etc/guard and /var/lib/guard; remove them by hand if unused."
+fi
+
+exit 0