servewaf answers rproxy's question about each request and takes its report (closes #170)

`smallwebwaf servewaf` runs today's checks on the visitor's request a
question carries and answers allow, deny or close, instead of passing it
to an app. rproxy's report then counts the bytes toward the byte limits
and adds the request to the client's history. SPEC.md gives the exact
question, answer and report, which rproxy follows. `smallwebwaf` with no
command is unchanged.

Judgement call: a third disposition, close, since closing the question's
connection would read as servewaf down and let the request through.
Judgement call: allow can answer a new request id, since a visitor behind
a trusted proxy can choose its own.

Model: opus-5-5

docs/alerts.md

@@ -78,6 +78,9 @@ on one line:
 
 - `instance`: `SWWAF_INSTANCE_NAME`. `time`: when the alert was raised. `event`:
   one of the events above.
+- `host`: the site of the request that raised the alert. Only `servewaf` sends
+  this field, since it serves many sites, and not for `source_failure`,
+  `file_error` and `summary`.
 - `client`: the address whose request raised the alert. Empty for
   `source_failure`, `file_error` and `summary`.
 - `netblock`: empty for `source_failure`, `file_error` and `summary`; otherwise
@@ -110,7 +113,7 @@ Slack and ntfy get a message with a title and a text. The title is the instance
 and the event, such as `fsn1app1/gitea: ban`. The text is the `reason`, then one
 line for each of these that the alert has:
 
-- the client, the netblock and the country;
+- the host, the client, the netblock and the country;
 - the file, the source, the error and the mode from `detail`;
 - the suppressed repeats, if any.