sysctl --system ignores missing /etc/sysctl.conf

sysctl --system ignores missing /etc/sysctl.conf

sysctl --system would not correctly return the RC for files in
subdirectories and would insist on having /etc/sysctl.conf

This update makes two changes when using sysctl --system:
  - The RC status is ORed for each config file, meaning an error in
    any file is propated to the RC
  - If /etc/sysctl.conf doesn't exist we just don't load it

References:
  https://bbs.archlinux.org/viewtopic.php?id=170005
  http://www.freelists.org/post/procps/wrong-defaults-for-sysctl-on-arch-linux