Set COLUMNS to MuttIndexWindow->cols in mutt_create_filter_fd. (closes #3837)

Set COLUMNS to MuttIndexWindow->cols in mutt_create_filter_fd. (closes #3837)

This allows filters to automatically (or via a command line argument)
adjust to the width of the pager/index.  This is even more important
with the sidebar, since the width of the pager is smaller than the
terminal width.

Thanks to Vincent Lefèvre for his helpful information and suggestions,
and to Richard Russon for the initial patches.