SWW_APPS: an app's Core Rule Set fixes apply only to the hostnames that serve it (closes #220)

Each app's fixes are one .conf file and one entry in apps. They apply
only while Inspect has set the app's transaction variable, such as
tx.app_gitea. SWW_APPS names each hostname's apps. A hostname gets only
its apps' fixes; while SWW_APPS is unset, every hostname gets every
app's fixes.

A hostname in SWW_APPS that is not a DNS name, such as *.example.com,
stops the start. Unset, SWW_APPS is logged at start as null; set
empty, as "".

Model: opus-5-5

internal/coreruleset/mastodon_test.go

@@ -57,7 +57,21 @@ const (
 func TestOrdinaryMastodonRequestsPass(t *testing.T) {
 	t.Parallel()
 
-	wantRecordedRequestsPass(t, "mastodon-requests.txt")
+	wantRecordedRequestsPass(t, "mastodon-requests.txt", "mastodon")
+}
+
+func TestMastodonFixesApplyOnlyToAHostnameThatServesMastodon(t *testing.T) {
+	t.Parallel()
+
+	crs := servingApps(t, map[string][]string{
+		giteaHost: {gitea}, otherHost: {"mastodon"},
+	})
+
+	// A search, which only Mastodon's fixes let through.
+	search := get("/api/v2/search?q=python3")
+
+	wantResult(t, crs, withHost(search, otherHost), coreruleset.Result{})
+	wantResult(t, crs, withHost(search, giteaHost), matched(932260))
 }
 
 // mastodonQueryFields sends each field that skips 930120, 932120, 932250