Enusure that my_global.h is included first

- Added sql/mariadb.h file that should be included first by files in sql
  directory, if sql_plugin.h is not used (sql_plugin.h adds SHOW variables
  that must be done before my_global.h is included)
- Removed a lot of include my_global.h from include files
- Removed include's of some files that my_global.h automatically includes
- Removed duplicated include's of my_sys.h
- Replaced include my_config.h with my_global.h

plugin/auth_gssapi/gssapi_errmsg.cc

@@ -26,8 +26,10 @@ ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 POSSIBILITY OF SUCH DAMAGE.
 */
 
+#include <my_global.h>
 #include <gssapi.h>
 #include <string.h>
+
 void gssapi_errmsg(OM_uint32 major, OM_uint32 minor, char *buf, size_t size)
 {
   OM_uint32 message_context;