main: bump fs.nr_open + fs.max-file to their largest possible values

main: bump fs.nr_open + fs.max-file to their largest possible values

After discussions with kernel folks, a system with memcg really
shouldn't need extra hard limits on file descriptors anymore, as they
are properly accounted for by memcg anyway. Hence, let's bump these
values to their maximums.

This also adds a build time option to turn thiss off, to cover those
users who do not want to use memcg.