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_.

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

internal/coreruleset/drupal.conf

@@ -4,10 +4,10 @@
 # request, before the rules it switches off, so this file is loaded before the
 # Core Rule Set's rules. Each comment names a request a rule refused. An
 # exclusion on a field of a body applies only when the query has no parameter
-# of that name, so it matters only while SWW_WAF_BODY_LIMIT is set. The
-# exclusions above the comment "The fields below are in bodies" also cover a
-# body field of the same name whatever the query holds, and only those below it
-# need a query without that name.
+# of that name, so it matters only while SWW_WAF_CORE_RULE_SET_BODY_LIMIT is
+# set. The exclusions above the comment "The fields below are in bodies" also
+# cover a body field of the same name whatever the query holds, and only those
+# below it need a query without that name.
 #
 # Each path may start with a language prefix, such as /fr in
 # /fr/admin/content. The prefix is two or three letters, then optionally a