Revised features section

This changes it to a bit vector. Brickbats to me.

5.2.3.24. Features

   (N octets of flags)

   The features subpacket denotes which advanced OpenPGP features a
   user's implementation supports. This is so that as features are
   added to OpenPGP that cannot be backwards-compatible, a user can
   state that they can use that feature.

   This subpacket is similar to a preferences subpacket, and only
   appears in a self-signature.

   An implementation SHOULD NOT use a feature listed when sending to a
   user who does not state that they can use it.

   Defined features are:

       First octet:

       0x01 - Modification Detection (packets 18 and 19)

   If an implementation implements any of the defined features, it
   SHOULD implement the features subpacket, too.