Minor cleanups/fixes.

 * Return ERANGE if the duration is over the max for UPnP leases.
 * Fix the format string for the spew when DeletePortMapping fails.
 * Add a undocumented command line flag that allows NAT-PMP mapping
   removal.  Not generally a "safe" thing to do, because the codebase
   that led me to disable it is widely deployed in the wild.