Add support for IMP_LOCALNODE_FORCE environment variable to allow working through NAT systems

   In a NAT (Network Address Translation) system, the IMP_LOCALNODE
   environment variable helps only when the local machine does not have
   an IP address.  If the machine does have an IP address, but it is not
   what the outside world knows the machine as (e.g. it is inside a NAT
   system), then the IP address encoded as the machine address is not
   reachable, and the machine outside the NAT system can't connect back.
   So a new environment variable is now supported by
   impz_unic.c:ImpZIPAddress() - IMP_LOCALNODE_FORCE.  If this is set, then
   it is translated first and will used as the IP address of the local
   machine (including for creating scratch files in IMP_SCRATCH), regardless
   of the actual IP address of the machine.  If set to the external IP
   address of the NAT router, communications outside of the NAT system
   will work.  But, at the same time, communications to other machines within
   the NAT system may fail.

   It should be noted that in order for communications through NAT to work,
   the IMP User port (see $IMP_DEV/impuserport) must be forwarded through
   the NAT router to the machine on which the internal task is running.  This
   is a security issue if the NAT router external IP is on the public
   internet, but probably ok if we are talking about a Virtual Machine
   within a private network.