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. Its constructor is New, like
rproxy's. 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