On 32 bit systems, it may be necessary to tweak this down to get high
numbers of worker threads squeezed into the address-space.
    
I have no idea how much stack-space a worker thread normally uses, so
no guidance is given, and we default to the system default.
    
Fixes #572



git-svn-id: http://www.varnish-cache.org/svn/branches/2.0@4402 d4fa192b-c00b-0410-8231-f00ffab90ce4