(read_crypttab) try harder to get enough data ({dm_active})
(idea from Colin, mga#5661)

thus enabling to choose to/from which crypttab file read/write-ing

needed for next changes (mga#5661)

As /dev/ is bind mounted into the prefix, we should always refer to /dev/ directly.
In particular this is needed inside wild_device.pm which is often used before the
prefix is even ready (i.e. when configuring existing partitions).

A specific problem here was detecting the major/minor numbers of partitions
which failed because the device node was being searched for inside the prefix
but we'd not even unlocked the encrypted / partition yet to mount it... catch 22.

mga#5661

condition got inverted in commit r3694 on Mar 23 2012
("perl_checker cleanups")

spot by Colin