This checking fixes the statcache bug that stopped NetBench from running

This checking fixes the statcache bug that stopped NetBench from running
correctly. Added new parameter "stat cache size" - set to 50 by default.

I now declare the statcache code officially "open" for business :-).
It gets a hit rate of 97% with a NetBench run and seems to make
using a case insensitive run as efficient as a case sensitive run.

Also tidied up our sys_select usage - added a maxfd parameter and
also added an implementation of select in terms of poll(), for systems
where poll() is much faster. This is disabled by default.

Jeremy.