rproxy, the proxy in front of each app, asks servewaf and fails open (closes #171)

`smallwebwaf rproxy` reads the first 64 KiB of each body, asks servewaf, and
passes what servewaf allows to the app within its own size and time limits.
It reports each request servewaf answered. SWRP_DEFAULT_DISPOSITION decides
when servewaf gives no answer.

Duplication: internal/rproxy copies passing-on code from internal/proxy until
#172 removes the originals.
Rule suppressed: gosec G704 on servewaf's URL, a setting.
Rule suppressed: gosec G705 on the answer written to the visitor.
Judgement call: the SWRP_ size and time limits take off; SWRP_TIMEOUT_MS does not.

Model: opus-5-5

docs/admin-endpoints.md

@@ -51,3 +51,9 @@ curl -H "Authorization: Bearer $TOKEN" \
 
 `smallwebwaf servewaf` answers these endpoints at its own address. No check runs
 on those requests (see "`servewaf`" in [`SPEC.md`](../SPEC.md)).
+
+## `rproxy`
+
+`smallwebwaf rproxy` answers only the health check, at its own address. It asks
+`servewaf` about any other request under `/_smallwebwaf/` (see "`rproxy`" in
+[`SPEC.md`](../SPEC.md)).