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/nextcloud.conf

@@ -5,8 +5,9 @@
 # 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 JSON body
 # applies only when the query has no parameter of that name, so it matters only
-# while SWW_WAF_BODY_LIMIT is set. Nextcloud's pages are matched at the site
-# root and under /index.php/, where a Nextcloud without pretty URLs serves them.
+# while SWW_WAF_CORE_RULE_SET_BODY_LIMIT is set. Nextcloud's pages are matched
+# at the site root and under /index.php/, where a Nextcloud without pretty URLs
+# serves them.
 #
 # Four rules read a search or a name as a command or a system file:
 #   930120: .htaccess