graphs: style: use left justification for states

graphs: style: use left justification for states

The label text of nodes is centered "by default" (by the use of '\n' as
a line feed). This gives bad readability to the grammar rules shown in
state nodes, a left justification is much nicer. This is done by using '\l'
as the line feed.

In order to allow \l in the DOT file, changes to the quoting system seem
necessary.

* src/print_graph.c (print_core): Escape tokens here, instead of...
* src/graphviz.c (output_node): Here...
(escape): Using this, new.

Signed-off-by: Akim Demaille <akim@lrde.epita.fr>