[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