Do not bind to same interface more than once. Fixes #328

Before 6ba9aa2771adcf785fe3fde03cd71832db15b086, multiple -l arguments
were ignored, and the last option passed was the one actually used.

This changes the behaviour to silently ignore duplicate -l flag options
passed and not crash and burn on the bind() call.

https://code.google.com/p/memcached/issues/detail?id=328