style: rename spec_defines_file as spec_header_file

The variable spec_defines_file denotes the name of the generated
header.  Its name is derived from --defines/%defines, whose name in
turn is derived from the fact that the header, in Yacc, contained the

Not only does the header now contain a lot more than just the token
definitions, but we no longer even generate macros, but an enum...

Let's modernize our vocabulary.

* src/files.h, src/files.c (spec_defines_file): Rename as...
(spec_header_file): this.

src/parse-gram.y

@@ -313,7 +313,7 @@ prologue_declaration:
 | "%defines" STRING
     {
       defines_flag = true;
-      spec_defines_file = xstrdup ($2);
+      spec_header_file = xstrdup ($2);
     }
 | "%error-verbose"                 { handle_error_verbose (&@$, $1); }
 | "%expect" INT                    { expected_sr_conflicts = $2; }