Some code and documentation improvements.

* data/c.m4 (b4_table_value_equals): New macro to capture
some repeated code.
* data/glr.c (yyis_pact_ninf): Use it here.
(yyis_table_ninf): Likewise.
(yyreportSyntaxError): Improve internal comments.
* data/yacc.c (yyis_pact_ninf): New macro copied from glr.c.
Use it everywhere possible.
(yyis_table_ninf): Likewise.
(yysyntax_error): Improve internal comments.
* data/lalr1.cc (yysyntax_error_): Likewise.
* data/lalr1.java (yysyntax_error): Likewise.
* src/tables.h: Improve comments about yypact, yytable, etc.
(cherry picked from commit 8741288212)

Conflicts:

	data/lalr1.java
	data/yacc.c

data/lalr1.java

@@ -691,7 +691,8 @@ m4_popdef([b4_at_dollar])])dnl
 	    StringBuffer res;
 
 	    /* Start YYX at -YYN if negative to avoid negative indexes in
-	       YYCHECK.  */
+	       YYCHECK.  In other words, skip the first -YYN actions for this
+	       state because they are default actions.  */
 	    int yyxbegin = yyn < 0 ? -yyn : 0;
 
 	    /* Stay within bounds of both yycheck and yytname.  */