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)

docs/state-files.md

@@ -1,13 +1,13 @@
 # State files
 
-The five JSON files in `SWW_STATE_DIR`, and what each holds. "Persistent state"
-in [`SPEC.md`](../SPEC.md) says how they are written, read and edited. Each has
-a top-level `version` of 1.
+The five JSON files in `SWW_WAF_STATE_DIR`, and what each holds. "Persistent
+state" in [`SPEC.md`](../SPEC.md) says how they are written, read and edited.
+Each has a top-level `version` of 1.
 
 ## `bans.json`
 
-Active and past bans: every ban an admin made, and up to `SWW_MAX_BANS` of those
-`discriminator` made. Per entry:
+Active and past bans: every ban an admin made, and up to `SWW_WAF_MAX_BANS` of
+those `discriminator` made. Per entry:
 
 - `netblock`, `start` and `expires` (`null` for a permanent ban);
 - `cause`: `attack`, `limit`, `admin` or `crowdsec`;
@@ -93,10 +93,10 @@ GeoJS answers, one per line:
 ## Sizes
 
 - `clients.json`: about 1 KiB per client, about 20 MiB at the default
-  `SWW_MAX_TRACKED_CLIENTS`. Written every 15 minutes, that is under 2 GiB of
-  disk writes a day.
+  `SWW_WAF_MAX_TRACKED_CLIENTS`. Written every 15 minutes, that is under 2 GiB
+  of disk writes a day.
 - `bans.json`: about 1.2 KiB per ban, at most about 2.5 KiB, so about 6 MiB at
-  the default `SWW_MAX_BANS`. Written every 15 minutes, a full file comes to
+  the default `SWW_WAF_MAX_BANS`. Written every 15 minutes, a full file comes to
   about 600 MiB of writes a day, and a hundred new bans a day add about 600 MiB
   more.
 - `lookups.json`: about 150 bytes per answer, under 1.5 GiB of writes a day when