nat: getopt returns "static" data no need to copy it

I used strdup in case the data would not be valid from after
the call to getopt and this creates a potential leak if a user
is specifying multiple configuration files. If I depend on the
fact that the string is a pointer into the argv[] array I can
kill the strdup and fix the unlikely leak.

Fixes: Coverity CID 1206578
1 file changed
tree: d72285138135a43cf6c1e915d7869ca0b38b6ea5
  1. debian/
  2. hlrsync/
  3. linux-kernel/
  4. openbsc/
  5. wireshark/
  6. README