upnpctrl: don't open mappings by default

The UPnPContext class currently has hardcoded limits on the number of
mappings that need to be "available" at any given time. This patch adds
the ability to modify these limits and sets them to zero in the upnpctrl
tool. This changes the behavior of the tool in two ways:
1) setting the minimum limit to zero means that no mappings are opened
   when upnpctrl starts;
2) setting the maximum limit to zero means that the "close" command will
   always cause the relevant mapping to be closed immediately instead of
   (potentially) being put back in the list of available mappings.

Change-Id: Idcf4262ce80a8b39f19ce7d3a201b40b3ea56911