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_.
Every place a name appeared changed: the code, the viper keys, tests,
documents, examples, the README's settings table and the image files.

Model: deepseek-v4-pro

internal/discriminator/client.go

@@ -43,7 +43,7 @@ func scheme(r *http.Request, isQuestion bool) string {
 
 // clientGroup is the client a request is counted toward: its IPv4
 // address, or its IPv6 group, the netblock its IPv6 address is in of the
-// length SWW_IPV6_GROUP_PREFIX sets, a /64 by default, since one abuser
+// length SWW_WAF_IPV6_GROUP_PREFIX sets, a /64 by default, since one abuser
 // usually holds a whole /64. An IPv4 address in IPv6 form counts as IPv4.
 func (h *handler) clientGroup(addr netip.Addr) netip.Prefix {
 	addr = addr.Unmap()