Rename discriminator to waf everywhere (closes #312)

The command is now `sww waf`, and the package is `internal/waf`. Types,
functions, files, tests, comments, docs, the Dockerfile, the runit
script, `script/`, compose examples, the log field `waf_line` and the
`X-From-Waf` test header follow. No behaviour changes, and no setting
changes name: rproxy already finds the waf through `SWRP_WAF_BACKEND`.

Done with one `sed -i` (Discriminator and DISCRIMINATOR to WAF,
discriminator to waf) over the files `git grep -i discriminator`
listed, as sneak allowed for this rename, then `git mv` for the
package and files. By hand afterwards: the header's spelling, which
the linter requires, and the padding after `waf` in `sww`'s usage
text.

Model: opus-5-5

internal/alerts/alerts.go

@@ -151,7 +151,7 @@ type Params struct {
 // Alert is one alert, as the webhook is sent it and alerts.json holds it,
 // with the fields of "The JSON object" in docs/alerts.md. ASN, ASName and
 // Country are, for a ban, the client's as the ban's notes give them. Host is
-// the site of the request that raised the alert, while one discriminator serves
+// the site of the request that raised the alert, while one waf serves
 // many sites, and "" otherwise.
 //
 //nolint:tagliatelle // docs/alerts.md names the alert's fields in snake_case