Rule files, and bans for a clear sign of attack (closes #24)

Every *.rules file in SWWAF_RULES_DIR is read at start and on each
change. Each request is checked against the rules after the rate
limits: log notes a match, block refuses with 403, ban refuses and bans
the netblock for SWWAF_ATTACK_BAN_DURATION, made permanent by its next
request or attack. path, query and uri are matched as the request line
sent them; header:Host and header:Transfer-Encoding are refused. Bans
gain a cause. The image ships 00-default.rules.

Judgement call: a header sent twice is matched with its values joined
by ", ".
Judgement call: SWWAF_MAX_BAN_DURATION does not cap a ban for an attack.
Not in this unit: offences for rule matches, with the error burst.

Model: opus-5-5

script/run

@@ -1,7 +1,9 @@
 #!/bin/sh
 # script/run: build bin/smallwebwaf with script/build and run it, with
 # the settings in the environment. Unless SWWAF_STATE_DIR is set, the
-# state files go in bin/state, beside the binary.
+# state files go in bin/state, beside the binary, and unless
+# SWWAF_RULES_DIR is set, the rule files are those of share/rules.d,
+# which the image ships.
 set -eu
 
 SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd -P)"
@@ -14,6 +16,10 @@ main() {
         export SWWAF_STATE_DIR
         mkdir -p "$SWWAF_STATE_DIR"
     fi
+    if [ -z "${SWWAF_RULES_DIR+set}" ]; then
+        SWWAF_RULES_DIR="$ROOT/share/rules.d"
+        export SWWAF_RULES_DIR
+    fi
     exec "$ROOT/bin/smallwebwaf"
 }