Be more aggressive about mmap'ing memory. The size_t thing is a bogus

Be more aggressive about mmap'ing memory.  The size_t thing is a bogus
constraint in FreeBSD and we shouldn't really listen to it.

On my laptop I can mmap 2422MB file this way.

This may be so aggressive that it leaves insufficient address space for
malloc/threadstacks and other issues.

If we find a way to pick up a platform/architecture specific limit, we
can enforce that along the way.

For now people can just specify a saner and smaller "-sfile" size.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@177 d4fa192b-c00b-0410-8231-f00ffab90ce4