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/sww/sww.go

@@ -51,7 +51,7 @@ const usage = `usage: sww <command>
 
 commands:
   rproxy          the reverse proxy in front of one app
-  waf   decides for rproxy, and keeps bans, counters and history
+  waf             decides for rproxy, and keeps bans, counters and history
   healthcheck     the container's health check
 `