Compute the number of extra comment starter characters.
(Extra semicolons in Lisp mode, extra stars in C mode, etc.)
If ARG is non-nil, just follow ARG.
If the comment starter is multi-char, just follow ARG.
Otherwise obey `comment-add'. (fn ARG)
How many more comment chars should be inserted by `comment-region'.
This determines the default value of the numeric argument of `comment-region'.
The `plain' comment style doubles this value.
This should generally stay 0, except for a few modes like Lisp where
it is 1 so that regions are commented with two or three semi-colons.