mcookie: use lib/randutils

The mcookie should reuse existing code, and there is definitely no need
to prefer /dev/random for this utility.  See reference for explanation
about later statement.

References: http://www.2uo.de/myths-about-urandom/
Signed-off-by: Sami Kerola <kerolasa@iki.fi>
mcookie: allow --file option be defined multiple times

Earlier only the last option argument took effect.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
mcookie: use same variable type consistently

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
mcookie: use control structure, and fix usage()

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
mcookie: add --max-size option

Just in case someone wants to add entropy from device with invocation
demonstrated below.

$ mcookie --file /dev/urandom --max-size 64k

Signed-off-by: Sami Kerola <kerolasa@iki.fi>