Renamed redisContext struct member 'unix' to 'unix_sock' to avoid encountering defined constant 'unix' in GNU C environment (see commit d8145d79).

Not all code using hiredis can compile using '-std=c99', and/or not all users are able to easily make that change to the build process of various open-source projects, so it is more pragmatic to choose a different identifier that does not impose this requirement.