Improved Cygwin FD_SETSIZE

Hello

I improved my previous pull request regarding FD_SETSIZE on Cygwin:

- The added test uses a custom sleep application instead of a less portable shell command.
- A commit message links to corresponding documentation in Cygwin's newlib and winsup sources
- The FD_SETSIZE is increased even further to 16384. This complies with the (arbitrary?) ">= NOFILE" requirement. And it's a value used by Cygwin sources. 

Kind regards, Paul