Bug 75816

From commit 66a5dbdf282435403f947c2caadd04bb0cdec752 which introduced requirements for /dev/* keyfiles to have .device services added, the documentation for handling passwordless truecrypt volumes is now incorrect. The suggested behaviour from `man crypttab` is to use /dev/null in the password field. After the above commit, a dev-null.device service is added as a requirement to mount the crypttab devices. This is incorrect as there is no need for a /dev/null device, nor does it exist (causing the boot to hang as it waits for the mount to timeout).

A temporary work around is to create a blank file and specify it in the password column and continue using the tcrypt-keyfile option. 

As an aside, the example at the bottom of `man crypttab` uses `/null' in the password column, I suspect this is incorrect too (and did not work for me).