[Remops] key size migration

[lists at notatla.org.uk]

> README_302.txt states a default keysize of 2048 bits.  My new node
> auto-generated a 4096 bit key without any size switch specified.

There are some minor changes since the first version as can be seen at
    http://www.zen19351.zen.co.uk/mixmaster302/
If you have the latest (3.0.2b) then
    2448 Nov  6 21:06 mixmaster-3.0.2b/doc/README_302
states the default at 4096 which is a change from
    2567 Oct 29 01:56 ./README_302.txt