Limit fill-column to 79

The current line-length limit is set to 80, but that allows a
character to appear in the 80th column, and that causes emacs to
display a line-wrap followed by a blank line when the display/window
width is 80 columns.  Furthermore, this seems to contradict the
coding-style rules on the wiki which suggest that the line limit
should be 79.

So reduce the line width in both the emacs control file and the
contrib vimrc file to 79 characters.

ChangeLog:

	* .dir-locals.el (c-mode): Change fill-column to 79.

contrib/ChangeLog:

	* vimrc (textwidth): Change non-gitcommit length to 79.