Factor socket-validity-checking stuff in ll2sock.h

Name rationale: ll2sock = long long 2 socket.

While there, also avoid using the EVUTIL_ prefix for things
that clearly belong to the NEUBOT_ prefix.

While there, roll out a better validity check on Win32: on MSDN
I have just read that a valid socket belongs to [0, x] where x
is (INVALID_SOCKET - 1).

Ref: http://msdn.microsoft.com/en-us/library/windows/desktop/ms740516%28v=vs.85%29.aspx