doctor: add package for running in-depth healthchecks; use in bugreport (#5413)

Change-Id: Iaa4e5b021a545447f319cfe8b3da2bd3e5e5782b
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>

ipn/localapi/localapi.go

@@ -221,6 +221,9 @@ func (h *Handler) serveBugReport(w http.ResponseWriter, r *http.Request) {
 	if note := r.FormValue("note"); len(note) > 0 {
 		h.logf("user bugreport note: %s", note)
 	}
+	if defBool(r.FormValue("diagnose"), false) {
+		h.b.Doctor(r.Context(), logger.WithPrefix(h.logf, "diag: "))
+	}
 	w.Header().Set("Content-Type", "text/plain")
 	fmt.Fprintln(w, logMarker)
 }