Patch: add configure-option for filedescriptors-number

Here's the (promised? threatened?) stab at the filedescriptors-number
problem. The attached patch adds a --with configure-option to explicitly
force the number of filedescriptors.
Please review and if satisfied apply. It's against 2.5-current, but it
should be seamless with any recent squid release.

-- 
	kinkie (kinkie-ml [at] libero [dot] it)
	Random fortune, unrelated to the message:
Quod erat demonstrandum.
	[Thus it is proven.  For those who wondered WTF QED means.]