Rework file descriptor handling in cnid_metad and cnid_dbd. We now pass

Rework file descriptor handling in cnid_metad and cnid_dbd. We now pass
the read end from socketpair() and the first client file descriptor
via the command line to cnid_dbd. That way there is no confusion with
descriptors 0 and 1 and the -d flag to cnid_metad is inheritited by cnid_dbd
(stdin/stdout/stderr remain open and useable). Also, we try to close all
descriptors not needed by cnid_dbd.