src: abstract installation paths

Lets abstract installation paths, so fw-admin can run in different trees:
 * /usr/local/
 * /usr/sbin/

With config files under:
 * /usr/local/etc/
 * /etc

With libs at:
 * /usr/local/lib
 * /usr/local/share
 * /usr/share
 * /usr/lib

Signed-off-by: Arturo Borrero <aborrero@cica.es>