Read every setting through viper and rename the settings (closes #211)

Every setting of discriminator and rproxy is read through viper, in place
of the os.LookupEnv function Main passed down. NewViper turns AutomaticEnv
and AllowEmptyEnv on, so a setting that is set but empty reads as set. A
setting may also be given as a file, the _FILE form, which rproxy's
settings now have too; rproxy still writes nothing to disk.

The settings are renamed. Discriminator's take the prefix SWW_WAF_,
rproxy's take SWW_RPROXY_, and the shared SWW_COMMAND keeps SWW_. The Core
Rule Set's settings take SWW_WAF_CORE_RULE_SET_ rather than SWW_WAF_WAF_.
Every place a name appeared changed: the code, the viper keys, tests,
documents, examples, the README's settings table and the image files.

Model: deepseek-v4-pro

docs/request-log.md

@@ -44,7 +44,7 @@ Every one of these lines has:
 - `time`: when `discriminator` had read the question's body, RFC 3339 with
   milliseconds, in UTC. The `report` line gives the same time as the `decision`
   line.
-- `instance`: `SWW_INSTANCE_NAME`.
+- `instance`: `SWW_WAF_INSTANCE_NAME`.
 - `client_ip`: the visitor's address, as `rproxy` worked it out.
 - `method`, `path` and `query`: the visitor's request.
 - `scheme`: how the visitor reached `rproxy` or the trusted proxy in front of
@@ -82,8 +82,8 @@ Every one of these lines has:
     - GeoJS had not answered by the time `discriminator` answered.
 - `content_type`: the visitor's `Content-Type`.
 - `content_length`: the body length the visitor announced.
-- `request_headers`: the headers listed in `SWW_LOG_REQUEST_HEADERS`, by name in
-  lower case.
+- `request_headers`: the headers listed in `SWW_WAF_LOG_REQUEST_HEADERS`, by
+  name in lower case.
 - `has_authorization` and `has_cookie`: true when the request had those headers.
   Their values are never logged.
 
@@ -96,8 +96,8 @@ Every one of these lines has:
 
 - `action`: what was done:
     - `forward`: answered `allow`, so that `rproxy` passes it to the app;
-    - `denied`: refused for `SWW_DENY_NETS`, or for a reputation source with the
-      action `deny`;
+    - `denied`: refused for `SWW_WAF_DENY_NETS`, or for a reputation source with
+      the action `deny`;
     - `banned`: refused under a ban, or for a `ban` rule, a trap path or a
       CrowdSec decision, each of which bans the client;
     - `country_denied`, `rate_limited`, `rule_blocked` and `waf_blocked`:
@@ -191,13 +191,13 @@ these meanings where they differ:
   sooner. It includes the wait for `discriminator`.
 - `duration_response`: milliseconds from then until the end of the answer.
 - `disposition`: what was done with the request: `allow`, `deny` or `close`. It
-  is `discriminator`'s answer, or the value of `SWRP_DEFAULT_DISPOSITION`. It is
-  left out when:
+  is `discriminator`'s answer, or the value of `SWW_RPROXY_DEFAULT_DISPOSITION`.
+  It is left out when:
     - `discriminator` was not asked, as for the health check;
     - the visitor went away while `discriminator` was being asked;
     - `discriminator`'s answer had no valid `X-Sww-Disposition`;
     - `rproxy` could not pass on `discriminator`'s answer, and answered `502`.
-- `default_disposition`: `true` when `SWRP_DEFAULT_DISPOSITION` gave
+- `default_disposition`: `true` when `SWW_RPROXY_DEFAULT_DISPOSITION` gave
   `disposition`, and otherwise left out.
 
 For example: