MINOR: limits: add is_any_limit_configured

MINOR: limits: add is_any_limit_configured

Let's encapsulate the check of all supported for now process internal limits in
a separate function. This will help in cases, when we need to simply check if
we have even only one limit set in the configuration file. It's important, as
the default value for a one limit (fd-hard-limits, for example) sometimes must
not affect the computation of the others.