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

Every setting of waf 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: waf'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_.

Model: deepseek-v4-pro (first version); opus-5-5 (rework)

Dockerfile

@@ -221,12 +221,12 @@ 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. At each start of waf, the run
-# script makes SWW_STATE_DIR if it is missing and gives it to the sww user.
+# script makes SWW_WAF_STATE_DIR if it is missing and gives it to the sww user.
 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