[wlanwatch] fix compiler warnings-as-errors on gcc-6.4 mips

[wlanwatch] fix compiler warnings-as-errors on gcc-6.4 mips

* argc/argv are currently unused
* msglen is currently unused
* "default" is a const buffer, but char *cp isn't, so
  change default string to be a non-const global string variable
* Make 'cp' private to each context that's using it, which fixes
  a "variable shadows previous declaration" warning and makes it
  easier to track where it was being leaked between address family
  sections