OpenBSD support and avoid using an identifier that may conflict.

This commit adds OpenBSD support for WalCommander.
It also changes the name of an identifier that is deemed a "Bad Identifier" that "conflicts and should be avoided." See https://gcc.gnu.org/onlinedocs/libstdc++/manual/source_code_style.html where _N is on the list so I changed it to _NL. There's no functional change there.
Better:

#elif defined( __FreeBSD__ ) || defined( __OpenBSD__ )