cmdgen: option to specify the random number device to use.

cmdgen: option to specify the random number device to use.

E.g. you might pass '--random-device=/dev/urandom'.

Mostly because I got sick of waiting for /dev/random to finish
blocking while I was trying to generate throwaway keys for testing bug
fixes in cmdgen itself. But it might also be useful on systems that
call their random device by a different name that we haven't
encountered.

(Since cmdgen also reads the saved PuTTY random seed file, setting
this option to /dev/zero will not render key generation deterministic.
It's tempting to provide _some_ way to do that, for testing purposes
and clearly marked as dangerous of course, but I think it would take
more faff than this.)