rnd: add gen_key_bytes to generate key from good entropy source

Make this a function and do not have this in curvetun hard coded.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
ioops: Add {read,write}_blob_or_die for reading/writing binary blobs

We need those functions for later key generation simplification.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>