ipn/localapi: fix inability to receive taildrop files w/ escaped names

The localapi was double-unescaping: once by net/http populating
the URL, and once by ourselves later. We need to start with the raw
escaped URL if we're doing it ourselves.

Started to write a test but it got invasive. Will have to add those
tests later in a commit that's not being cherry-picked to a release
branch.

Fixes #2288

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/localapi/localapi.go

@@ -266,7 +266,7 @@ func (h *Handler) serveFiles(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "file access denied", http.StatusForbidden)
 		return
 	}
-	suffix := strings.TrimPrefix(r.URL.Path, "/localapi/v0/files/")
+	suffix := strings.TrimPrefix(r.URL.EscapedPath(), "/localapi/v0/files/")
 	if suffix == "" {
 		if r.Method != "GET" {
 			http.Error(w, "want GET to list files", 400)