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

Dockerfile

@@ -175,12 +175,12 @@ RUN groupadd --system --gid 65532 sww \
         --gid sww --no-create-home --shell /usr/sbin/nologin \
         sww
 
-# The state files' directory, SWW_STATE_DIR by default, where a volume
+# The state files' directory, SWW_WAF_STATE_DIR by default, where a volume
 # is mounted to keep them across deploys. The run script gives it to the
 # sww user at each start of discriminator.
 RUN mkdir /var/lib/sww
 
-# The default rule file, in SWW_RULES_DIR by default, where a Dockerfile
+# The default rule file, in SWW_WAF_RULES_DIR by default, where a Dockerfile
 # built FROM this image can copy rule files of its own beside it.
 COPY share/rules.d/00-default.rules /etc/sww/rules.d/00-default.rules