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)

internal/coreruleset/gitea.conf

@@ -5,7 +5,7 @@
 # Each comment names a request the rule refused.
 
 # Every rule below is skipped unless Inspect has set tx.app_gitea. It sets it
-# while SWW_APPS is unset, or lists gitea or forgejo for the request's hostname.
+# while SWW_WAF_APPS is unset, or lists gitea or forgejo for the request's hostname.
 # The skip reaches only phase 1, so a rule of a later phase must check
 # tx.app_gitea itself.
 SecRule &TX:app_gitea "@eq 0" \