Various fixes/tweaks to HTTP placeholder variables and file matching

- Rename http.var.* -> http.vars.* to be more consistent
- Prefixing a path matcher with * now invokes simple suffix matching
- Handlers and matchers that need a root path default to {http.vars.root}
- Clean replacer output on the file matcher's file selection suffix

modules/caddyhttp/reverseproxy/fastcgi/fastcgi.go

@@ -79,7 +79,7 @@ func (Transport) CaddyModule() caddy.ModuleInfo {
 // Provision sets up t.
 func (t *Transport) Provision(_ caddy.Context) error {
 	if t.Root == "" {
-		t.Root = "{http.var.root}"
+		t.Root = "{http.vars.root}"
 	}
 	return nil
 }