take time if there are hundreds of thousands of possible filedescriptors.

Instead do it once, right at startup in the manager process, and then
keep track of the fd's we use there, and have the child clean only
up to the max seen, with an allowance for filedescriptors held by
libraries (syslog, resolver, pidfiles etc)

Fixes	#699



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4648 d4fa192b-c00b-0410-8231-f00ffab90ce4