Avoid absolute path for privileged commands

Using absolute path for a privileged command is a bad idea as this path may vary.
For example /usr/sbin/brctl in Debian and /sbin/brctl in ALTLinux. Using $PATH is
a better idea.

Signed-off-by: Vitaly Kuznetsov <vitty@altlinux.ru>
Reviewed-by: Iustin Pop <iustin@google.com>
Signed-off-by: Iustin Pop <iustin@google.com>