main: when generating the resource limit to pass to children, take FD_SETSIZE into...

main: when generating the resource limit to pass to children, take FD_SETSIZE into consideration

When we synthesize a "struct rlimit" structure to pass on for
RLIMIT_NOFILE to our children, let's explicitly make sure that the soft
limit is not above FD_SETSIZE, for compat reason with select().

Note this only applies when we derive the "struct rlimit" from what we
inherited. If the user configures something explicitly it always takes
precedence.