Fix information leakage.
Without this fix it is possible to cheat policies like:

• sysctl security.bsd.see_other_[gu]ids=0,
• mac_seeotheruids(4),
• jail(2)

and get full processes list with their arguments.

This problem exists from revision 1.62 of kern_proc.c when it was
introduced.

Reviewed by: nectar, rwatson.