* NEWS: The -g and --graph options now output graphs in Graphviz

* NEWS: The -g and --graph options now output graphs in Graphviz
DOT format, not VCG format.
* doc/bison.1: Likewise.
* doc/bison.texinfo (Understanding, Bison Options): Likewise.
* TODO: Remove Graphviz entry.
* src/Makefile.am (bison_SOURCES): Add graphviz.c and graphviz.h;
remove vcg.c, vcg.h, vcg_defaults.h.
* src/vcg.c, src/vcg.h, src/vcg_defaults.h: Remove.
* src/graphviz.c, src/graphviz.h: New files.
* src/files.c (compute_output_file_names): Output .dot, not .vcg.
* src/files.h: Make comment more generic.
* src/main.c (main): Likewise.
* src/print_graph.h: Likewise.
* src/getargs.c (usage): Make usage description more generic.
* src/print_graph.c: Include graphviz.h rather than vcg.h.
(static_graph, fgraph): Remove.  All uses changed to pass
arguments instead of sharing a static var.
(print_core, print_actions, print_state, print_graph):
Output graphviz format rather than VCG format.
* tests/.cvsignore: Remove *.vcg; add *.dot.
* tests/output.at: Expect *.dot files, not *.vcg files.