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/wordpress.conf

@@ -3,7 +3,7 @@
 # the rules it switches off, so this file is loaded before the Core Rule Set's
 # rules. Each comment names a request the 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.
+# matters only while SWW_WAF_CORE_RULE_SET_BODY_LIMIT is set.
 
 # WordPress takes a call to its REST API at any of four addresses:
 #   /wp-json/wp/v2/posts/1