miniupnpd/natpmp.c: return correct error code when all external ports in use

Instead of returning code 3 ("Network Failure"), we should the
more appropriate code 4 ("Out of resources") when no external
port is available for a mapping.
Merge pull request #54 from razzfazz/fix_no_eport_error_code

miniupnpd/natpmp.c: return correct error code when all external ports in...