Respect line_length in vertical grid modes and deprecate mode 6

In the three vertical grid multi line modes (4, 5 and 6), each
mode had a bug where the line length would be overcounted or
undercounted by one character when used in combination with an
unexpected value for the include_trailing_comma setting.

This could cause import lines to exceed the line_length or be
wrapped at one less than the specified line_length.

Count the trailing characters correctly.

This also deprecates mode 6, since the only difference between
this mode and mode 5 is in how it handles include_trailing_comma.
The distinction is no longer relevant, since mode 5 can now handle
both possible values of include_trailing_comma.

For backwards compatibility, make mode 6 an alias for mode 5.