Read configuration files from /etc and /usr

Read configuration files from /etc and /usr

Add support for the so called "stateless" configuration pattern (read
from /etc, fall back to /usr), giving system administrators a way to
define local configuration without changing any distro-provided files.

In practice this means that each configuration file FOO is loaded
from /usr/lib/iproute2/FOO unless /etc/iproute2/FOO exists.

Signed-off-by: Gioele Barabucci <gioele@svario.it>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>