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/bans/bans.go

@@ -123,7 +123,7 @@ type Notes struct {
 	LimitPercentSetting string `json:"limit_percent_setting,omitempty"`
 	// RuleID and Target are, for a ban for a clear sign of attack, the id
 	// of the rule file rule that matched, and its target; TrapPath is, for
-	// one for a request for a path in SWW_TRAP_PATHS, that path as the
+	// one for a request for a path in SWW_WAF_TRAP_PATHS, that path as the
 	// client sent it.
 	RuleID   string `json:"rule_id,omitempty"`
 	Target   string `json:"target,omitempty"`
@@ -199,7 +199,7 @@ type Ledger struct {
 	// v4Lengths and v6Lengths are the lengths of the IPv4 and IPv6
 	// netblocks that have been banned. Check looks for a ban at each of
 	// them, so that a ban read from bans.json refuses every client in its
-	// netblock even when it was made with another SWW_BAN_SCOPE_V4_PREFIX,
+	// netblock even when it was made with another SWW_WAF_BAN_SCOPE_V4_PREFIX,
 	// or another length of an IPv6 client's netblock.
 	v4Lengths, v6Lengths []int
 }