rproxy: give its own answers at least 10 seconds to be sent (closes #295)

The write deadline of an answer rproxy gives itself was
SWRP_MAX_RESPONSE_TIME_MS from when the answer started. With a limit
under a second, a host too busy to run rproxy for that long passes the
deadline before the first byte is written. Go then closes the
connection and the visitor gets no answer.

Model: opus-5-5

docs/settings.md

@@ -436,7 +436,8 @@ A time here is a whole number of milliseconds above zero, such as `2000`.
   headers, the time until the app has been sent the whole request. A time or
   `off`.
 - `SWRP_MAX_RESPONSE_TIME_MS` (default `1800000`): how long the answer may take,
-  from the end of the request to its last byte reaching the visitor. A time or
+  from the end of the request to its last byte reaching the visitor. An answer
+  `rproxy` gives itself always has at least 10 seconds to be sent. A time or
   `off`.
 - `SWRP_LISTEN_ADDR` (default `:8080`): where `rproxy` listens, such as `:9000`.
 - `SWRP_UPSTREAM_URL` (default `http://127.0.0.1:8081`): the app. An `http` or