We used to try to terminate the managed proxy process even if it
failed while launching. We introduce a new managed proxy state, to
represent a *broken* and *not launched* proxy.
* Use strcmpstart() instead of strcmp(x,y,strlen(y)).
* Warn the user if the managed proxy failed to launch.
* Improve function documentation.
* Use smartlist_len() instead of n_unconfigured_proxies.
* Split managed_proxy_destroy() to managed_proxy_destroy()
and managed_proxy_destroy_with_transports().
* Constification.
