keys: Fix defaults and documentation.

keys: Fix defaults and documentation.

A small number of related changes.

  * Make the default encryption algorithm be Rijndael (AES) rather than
    Blowfish.  I think this is now the right recommendation to make,
    even if it's not my personal taste.  (I'm actually using Twofish
    nowadays anyway.)

  * Make the default field size for Schnorr groups be 3072 bits rather
    than 2048, which reflects the advice given by NIST and IEEE 1363.

  * Reformat the tripe-keys.master file in line with current thinking.

  * Fix a minor typo in tripe-keys.conf.5.in.