sysctl: add --system switch

instead of requiring distributions to construct a loop around sysctl
in boot scripts just scan a set of default directories if the --system
switch is used.

Config files are applied in alphabetic order of their base name.
Each base name is only applied once according to the directory
preference. /etc/sysctl.conf is always applied last.

Signed-off-by: Ludwig Nussel <ludwig.nussel@suse.de>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>