Make partially specified permanent links without `dst_addr'

Make partially specified permanent links without `dst_addr'
but with `dst_port' work for outgoing packets.

This case was not handled properly when I first fixed this
in revision 1.17.

This change is also required for the upcoming improved PPTP
support patches -- that is how I found the problem.

Before this change:

# natd -v -a aliasIP \
  -redirect_port tcp localIP:localPORT publicIP:publicPORT 0:remotePORT

Out [TCP]  [TCP] localIP:localPORT -> remoteIP:remotePORT aliased to
           [TCP] aliasIP:localPORT -> remoteIP:remotePORT

After this change:

# natd -v -a aliasIP \
  -redirect_port tcp localIP:localPORT publicIP:publicPORT 0:remotePORT

Out [TCP]  [TCP] localIP:localPORT -> remoteIP:remotePORT aliased to
           [TCP] publicIP:publicPORT -> remoteIP:remotePORT