Fix issue #356 - server.maxclients could be minus!

## Introduction
This is my attempt to fix issue #356.

## Tests
Unfortunately, I have no idea how to set limit on fd through TCL script. I did look it up on Google, but haven't found any useful link. Anyway, I did manual testing -- 

running: `ulimit -n 1024 && ./redis-server`; expecting to get

    Unable to set the max number of files limit to 10032 (Operation not permitted), 
    setting the max clients configuration to 912.

running: `ulimit -n 32 && ./redis-server`; expecting to get

    Unable to set the max number of files limit to 10032 (Operation not permitted), 
    setting the max clients configuration to 0.

running: `ulimit -n 16 && ./redis-server`; expecting to exit Redis with this message

    Unable to set the max number of files limit to the smallest files number (32) for
    running Redis (Operation not permitted)

## Notes
* Feel free to share your thoughts.