MINOR: init: add global setting "fd-hard-limit" to bound system limits

MINOR: init: add global setting "fd-hard-limit" to bound system limits

On some systems, the hard limit for ulimit -n may be huge, in the order
of 1 billion, and using this to automatically compute maxconn doesn't
work as it requires way too much memory. Users tend to hard-code maxconn
but that's not convenient to manage deployments on heterogenous systems,
nor when porting configs to developers' machines. The ulimit-n parameter
doesn't work either because it forces the limit. What most users seem to
want (and it makes sense) is to respect the system imposed limits up to
a certain value and cap this value. This is exactly what fd-hard-limit
does.

This addresses github issue #1622.