doc: prefer the ".gv" extension to ".dot"

See http://marc.info/?l=graphviz-devel&m=129418103126092 for the
motivation (basically, some word processor now uses *.dot).

* doc/figs/example-reduce.dot: Rename as...
* doc/figs/example-reduce.gv: this.
* doc/figs/example-shift.dot: Rename as...
* doc/figs/example-shift.gv: this.
* doc/figs/example.dot: Rename as...
* doc/figs/example.gv: this.
* doc/local.mk: Adjust.
build: ship the ASCII art figures

We don't ship the *.txt files that are used to build the info
file.
Reported by Colin Daley.

* doc/figs/example.txt: New.
* doc/local.mk (bison.info): Depend on the txt files.
And ship them.
symbols: improve symbol aliasing

Rather than having duplicate info in the symbol and the alias that has
to be resolved later on, both the symbol and the alias have a common
pointer to a separate structure containing this info.

* src/symtab.h (sym_content): New structure.
* src/symtab.c (sym_content_new, sym_content_free, symbol_free): New

* src/AnnotationList.c, src/conflicts.c, src/gram.c, src/gram.h,
* src/graphviz.c, src/ielr.c, src/output.c, src/parse-gram.y, src/print.c
* src/print-xml.c, src/print_graph.c, src/reader.c, src/reduce.c,
* src/state.h, src/symlist.c, src/symtab.c, src/symtab.h, src/tables.c:
Adjust.

* tests/input.at: Fix expectations (order changes).