[R6RS] Making continuations created by BEGIN accept multiple values

[R. Kent Dybvig]

With respect to the following proposals:

 1. continuations created by begin accept any number of values

 2. set!, write, and other forms and procedures that currently return
    unspecified values return zero values instead

I've looked back on the r5rs mailings, and it seems like people objected
to the first change because of some perceived implementation difficulty,
and they objected to the second because we weren't making the first
change.  If there is no longer a perceived implementation difficulty,
then let's go ahead with both changes.

Kent