tokens: properly define the YYEOF token kind

Currently EOF is handled in an adhoc way, with a #define YYEOF 0 in
the implementation file.  As a result, the user has to define her own
EOF token if she wants to use it, which is a pity.

Give the $end token a visible kind name, YYEOF.  Except that in C,
where enums are not scoped, we would have collisions between all the
definitions of YYEOFs in the header files, so in C, make it
<api.PREFIX>EOF.

* data/skeletons/c.m4 (YYEOF): Override its name to avoid collisions.
Unless the user already gave it a different name.
* data/skeletons/glr.c (YYEOF): Remove.
Use ]b4_symbol(0, [id])[ instead.
Add support for "pre_epilogue", for glr.cc.
* data/skeletons/glr.cc: Remove dead code (never emitted #undefs).
* data/skeletons/yacc.c
* src/parse-gram.c
* src/reader.c
* src/symtab.c
* tests/actions.at
* tests/input.at

data/skeletons/yacc.c

@@ -716,7 +716,6 @@ enum { YYENOMEM = -2 };
 #define yyerrok         (yyerrstatus = 0)
 #define yyclearin       (yychar = YYEMPTY)
 #define YYEMPTY         (-2)
-#define YYEOF           0
 
 #define YYACCEPT        goto yyacceptlab
 #define YYABORT         goto yyabortlab
@@ -1760,9 +1759,9 @@ yyread_pushed_token:]])[
       yychar = ]b4_lex[;]])[
     }
 
-  if (yychar <= YYEOF)
+  if (yychar <= ]b4_symbol(0, [id])[)
     {
-      yychar = YYEOF;
+      yychar = ]b4_symbol(0, [id])[;
       yytoken = ]b4_symbol_prefix[YYEOF;
       YYDPRINTF ((stderr, "Now at end of input.\n"));
     }
@@ -1957,10 +1956,10 @@ yyerrlab:
       /* If just tried and failed to reuse lookahead token after an
          error, discard it.  */
 
-      if (yychar <= YYEOF)
+      if (yychar <= ]b4_symbol(0, [id])[)
         {
           /* Return failure if at end of input.  */
-          if (yychar == YYEOF)
+          if (yychar == ]b4_symbol(0, [id])[)
             YYABORT;
         }
       else