add a "stat cache" boolean smb.conf option. (defaults to on)

add a "stat cache" boolean smb.conf option. (defaults to on)

I think we need this so we can rule out stat cache bugs when dealing
with bug reports. If we ask a user to disable the stat cache and the
problem persists then we know it isn't a stat cache bug. The stat
cache code is sufficiently complicated that it can be pretty hard to
tell if it is causing problems or not.