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/config/config_test.go

@@ -636,11 +636,11 @@ func TestInvalidCoreRuleSetSettingStopsTheStart(t *testing.T) {
 			wafBodyLimit, "128KB",
 			`"128KB" is not a size such as 512K, 100M or 5G, or off`,
 		},
-		{wafBodyLimit, "65K", `"65K" is more than 64K, the most discriminator ` +
+		{wafBodyLimit, "65K", `"65K" is more than 64K, the most waf ` +
 			`gets of a body`},
 		{
 			wafBodyLimit, "65537",
-			`"65537" is more than 64K, the most discriminator gets of a body`,
+			`"65537" is more than 64K, the most waf gets of a body`,
 		},
 	} {
 		t.Run(tc.name+"="+tc.value, func(t *testing.T) {