Force close on exec on first 256 file descriptors when launching a

slurmstepd to close potential open ones.

It was pointed out the slurmd using acct_gather_energy/ipmi links to
freeipmi which could possibly open /dev/ipmi0 without the close on exec
flag set as root while launching a step leaving it open in the users app.

What this does is sets the flag on the first 256 to mitigate the concern.

Reported by Maksym Planeta.

Bug 2506