style: avoid explicit symbol numbers

This should have been part of commit "symbols: stop dealing with YYEMPTY
as b4_symbol(-2, ...)" (cd40ec9526).
Give names to all the special symbols: "eof", "error" and "undef".

* data/skeletons/bison.m4 (b4_symbol): Let `b4_symbol(eof, ...)` mean
`b4_symbol(0, ...)`, `b4_symbol(error, ...)` mean `b4_symbol(1, ...)`,
and , `b4_symbol(undef, ...)` mean `b4_symbol(2, ...)`..

* data/skeletons/c.m4, data/skeletons/glr.c, data/skeletons/glr.cc,
* data/skeletons/glr2.cc, data/skeletons/lalr1.cc,
* data/skeletons/lalr1.d, data/skeletons/lalr1.java,
* data/skeletons/yacc.c:
Prefer symbols to numbers.

data/README.md

@@ -81,7 +81,12 @@ name, whether terminal, etc.), bison.exe defines one macro per (token,
 field), where field can `has_id`, `id`, etc.: see
 `prepare_symbols_definitions()` in `src/output.c`.
 
-The macro `b4_symbol(NUM, FIELD)` gives access to the following FIELDS:
+NUM can be:
+- `empty` to denote the "empty" pseudo-symbol when it exists,
+- `eof`, `error`, or `undef`
+- a symbol number.
+
+FIELD can be:
 
 - `has_id`: 0 or 1
   Whether the symbol has an `id`.