rbd, rbd-nbd: don't strip trailing newline in passphrase files

rbd, rbd-nbd: don't strip trailing newline in passphrase files

One of the stated goals is compatibility with standard LUKS tools,
in particular being able to load encryption on images formatted with
cryptsetup.  cryptsetup doesn't do this and this really interferes
with randomly generated (binary) passphrases.

While at it, open passphrase files as binary -- it communicates the
intent if nothing else on POSIX.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>