Replacing select and FD_SET with epoll to support more than 1024 fake switches

I replaced select and FD_SET functions in cbench.c to epoll to support more than 1024 fake switches.
I have tested it with 2048 switches and it works well.
Before run it, you need to set open files limits to more than 1024 using "ulimit -n 4096."
I set the __FD_SETSIZE to 4096 at /usr/include/bits/typesizes.h.
In case you need more than 4096 open connections, you can set the limits with a higher value.
I guess that depends on systems.
Nit - indentation. The cbench code uses 4 chars of space as indentation - your editor seems to use tab -  please reformat to four spaces for consistency.