Don't actively violate the system limit of maximum open files (RLIMIT_NOFILE).

This avoids irritating kernel logs (if system overstep violations are enabled)
and also the grsecurity alert when starting PostgreSQL.

original patch by Jacek Drobiecki

References:
http://archives.postgresql.org/pgsql-bugs/2004-05/msg00103.php
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=248967