Make the cmd_fre function in src/common/blib_func.c portable #275

Based on #273, the cmd_fre function in src/common/blib_func.c is not portable. The function assumes that _UnixOS implies Linux rather than Unix in general. The code doesn’t work on FreeBSD and most likely will not work on any other BSD either (this needs to be verified).

The goal is to make the function portable to support FreeBSD, initially, and later on support for other BSDs and Darwin/macOS as well.