!includedir reads files in random order

Currently !includedir directive in my.cnf seems to include files in some unspecified order. This makes it impossible to override settings in one specific include file by adding another file that is included later.

The server should include files in alphabetical order, so that one could use config files with conventional two-digit prefixes.

the fix is to add MY_WANT_SORT to my_dir() in my_default.c