Check ketwords against active keyword subset mask.

The begin_keywords directives manage a mask of keyword sets that are
active at a given moment. This patch makes the lexor actually test
the given keyword against the mask. If it matches, return the keyword
id, if it is disabled then it is an IDENTIFIER.

lexor_keyword.gperf

@@ -130,6 +130,8 @@ int lexor_keyword_code(const char*str, unsigned nstr)
       const struct lexor_keyword*rc = check_identifier(str, nstr);
       if (rc == 0)
 	  return IDENTIFIER;
+      else if ((rc->mask & lexor_keyword_mask) == 0)
+          return IDENTIFIER;
       else
 	  return rc->tokenType;
 }