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

README.md

@@ -688,8 +688,8 @@ aside as `<name>.bad`, and never stops the running process (see
   health check.
 - `internal/config`: reads the settings, the one place they are read.
 - `internal/rproxy`: the reverse proxy in front of one app.
-- `internal/proxy`: `servewaf`'s server, which runs the checks and answers its
-  own endpoints.
+- `internal/servewaf`: `servewaf`'s server, which runs the checks and answers
+  its own endpoints.
 - `internal/ratelimit`, `internal/bans`, `internal/rules`, `internal/waf`,
   `internal/lookup`, `internal/reputation`, `internal/anomaly` and
   `internal/alerts`: one check or source each.