[core] Unify unix macro

[y761823]

In Ubuntu 20.04, gcc/clang has only lowercase unix macro, no UNIX macro.
And in channel.cpp, there are both unix and UNIX macro, unify to unix

➜  ~ echo | clang-11 -dM -E - | grep -i unix                                    
#define __unix 1
#define __unix__ 1
#define unix 1
➜  ~ echo | g++-14 -dM -E - | grep -i unix
#define __unix 1
#define __unix__ 1
#define unix 1

[ethouris]

There are two problems with this PR.

1. We don't exactly know why there is the UNIX macro in use there. This actually stretches up to the times of the original codebase for SRT, UDT. The problem is, gcc has never been setting it (at least the oldest version I know) and all the information available on it is that one might at best have set it on their own. The build files (also those on UDT4) do not define it at all. So, the current state of things is that likely the UNIX macro was so far never defined in any build made by anyone for SRT. By changing it to unix you change the behavior of the existing builds and I doubt this was originally intended. For what I have seen there is that it uses exceptionally F_NDELAY flag in case when the system doesn't support the SO_RCVTIMEO socket option for UDP sockets. Changing UNIX into unix definitely isn't the right approach. There should be rather revised various versions for these conditions and see if there is any reason to have a different approach on different systems there, and how.
2. You have modified the select function call that was specifying so far the descriptor set the same for reading and error set by removing this for the error set. Is there any merit for this change? If so, please add appropriate description and explanation for this change.