bug #702 - ALTER TABLE tm UNION=(t1,t2) doesn't work if t1.MYD is compressed

myisammrg/myrg_open.c:
  never mark merge table as readonly even if all underlying myisam tables are readonly,
  otherwise ALTER TABLE merge UNION=(t1,t2,t3,...) will not work
sql/handler.cc:
  map EACCES to ER_OPEN_AS_READONLY.
  can only happen on modifying merge table that has (some) read-only myisam tables

sql/handler.cc

@@ -801,6 +801,9 @@ void handler::print_error(int error, myf errflag)
 
   int textno=ER_GET_ERRNO;
   switch (error) {
+  case EACCES:
+    textno=ER_OPEN_AS_READONLY;
+    break;
   case EAGAIN:
     textno=ER_FILE_USED;
     break;