sysctl: read settings from /etc/sysctl.d/*.conf

This changes makes it possible to store custom settings
in individual files inside the directory /etc/sysctl.d/.

Signed-off-by: Stefan Tomanek <stefan.tomanek+openwrt@wertarbyte.de>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46239 3c298f89-4303-0410-b956-a3cf2f4a3e73

package/base-files: change sysctl handling
Move /etc/init.d/sysctl to index 00 and add hotplug script that applies
interface specific sysctls for interfaces that only appear later - this
allows to reliably configure per-interface parameters in sysctl.conf, e.g.
to disable ipv6 autoconfig on a specific iface.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@36129 3c298f89-4303-0410-b956-a3cf2f4a3e73