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/mastodon.conf

@@ -4,8 +4,8 @@
 # exclusion runs with the 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.
+# has no parameter of that name, so it matters only while
+# SWW_WAF_CORE_RULE_SET_BODY_LIMIT is set.
 #
 # Gitea serves /api/v1/ too, and Drupal serves /admin/. So only Mastodon's own
 # resources under them count as Mastodon's paths.