bug#1153: default for comment-style not very good

[Dan Nicolaescu]

`comment-style' defaults to indent-or-triple.  

This makes a difference only for modes that use a single character
comment syntax, comment-region will use three characters to comment.
For modes like assembly, shell, perl, etc this is very unusual way of
commenting.  Even for elisp, triple ; is not the most used way of
commenting.  Hence this version should not be the default, a more
appropriate default would be 'indent.