Rename internal/proxy to internal/servewaf (closes #173)

The package passes nothing to an app since rproxy took that over, so it
is named after the command it serves. The rproxy tests call their
stand-in for servewaf standIn, since a local servewaf would hide the
package. No change of behaviour.

Model: opus-5-5

SPEC.md

@@ -66,10 +66,10 @@ to them.
       one it does not know, the binary prints the commands and exits with
       status 2.
     - `internal/config`: reads the settings, the one place they are read.
-    - `internal/proxy`: `servewaf`'s server. It runs the checks in `check`, and
-      holds `servewaf`'s question, answer and report. It answers `servewaf`'s
-      own endpoints in `answerOwnEndpoint`. It counts bytes, refusals and
-      anomalies, and writes the log lines.
+    - `internal/servewaf`: `servewaf`'s server. It runs the checks in `check`,
+      and holds `servewaf`'s question, answer and report. It answers
+      `servewaf`'s own endpoints in `answerOwnEndpoint`. It counts bytes,
+      refusals and anomalies, and writes the log lines.
     - `internal/rproxy`: the reverse proxy in front of one app that asks
       `servewaf` about each request. It passes a request to the app with the
       standard library's `httputil.ReverseProxy`, within the time and size