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/reputation/dnsbl_test.go

@@ -40,8 +40,8 @@ const (
 	// zone and otherZone are the DNSBL zones the tests name.
 	zone      = "dnsbl.example"
 	otherZone = "other.example"
-	// cacheTTL is the tests' SWW_REPUTATION_CACHE_TTL, and timeout their
-	// SWW_REPUTATION_TIMEOUT: a second, the least time /etc/resolv.conf
+	// cacheTTL is the tests' SWW_WAF_REPUTATION_CACHE_TTL, and timeout their
+	// SWW_WAF_REPUTATION_TIMEOUT: a second, the least time /etc/resolv.conf
 	// can have Go's resolver wait for one server, so that it is the
 	// DNSBL's own timeout that ends a query, whatever that file says.
 	cacheTTL = 24 * time.Hour
@@ -419,7 +419,7 @@ func TestNeitherAVerdictOfAZoneNotNamedNorOnePastItsTTLIsKept(t *testing.T) {
 	dnsbl := reputation.NewDNSBL(p)
 
 	// The last verdict still in use, one fetched a TTL ago, and one of a
-	// zone SWW_DNSBL_ZONES does not name.
+	// zone SWW_WAF_DNSBL_ZONES does not name.
 	inUse := reputation.Verdict{
 		Zone: zone, Client: netip.MustParseAddr(listed), Listed: true,
 		Fetched: now.Add(-cacheTTL + time.Nanosecond),