Secure file-checks from root to directory

[whotwagner]

We should add a check that makes sure that the path to a specific directory is secure. Let's say the persistency-dir is /var/lib/aminer. We should open / with O_PATH and check if this is secure(uid,gid,permissions,acl), and use the dir_fd to open /var and make the same checks and use this dir_fd to open /var/lib/aminer and make the same checks. This would garantee that the whole path to the given directory is secure against race-conditions.

[ernstleierzopf]

waiting for #474 to be accepeted before continuing here.