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/sysusers.d/guard.conf

@@ -0,0 +1,3 @@
+# Guard's system user (systemd-sysusers): it reads logs and talks to
+# Housecarl, nothing more.
+u guard - "Guard log watcher" /var/lib/guard /usr/sbin/nologin