Start populating mmc/mmc_hl_cmds.c. Create mmc directory in preparation

for making it a library.

mmc.h:  CDIO_MMC_GPCMD_ALLOW_MEDIUM_REMOVAL ->
        CDIO_MMC_GPCMD_PREVENT_ALLOW_MEDIUM_REMOVAL
mmc.c:  Move mm_eject_media and mmc_set_drive_speed to mmc/mmc_hl_cmds.c
mmc_ll_cmds.c: add mmc_prevent_allow_medium_removal,
               move mmc_mode_sense to mmc/mmc_hl_cmds.c

lib/driver/gnu_linux.c

@@ -35,6 +35,7 @@ static const char _rcsid[] = "$Id: gnu_linux.c,v 1.33 2008/06/25 07:46:21 rocky
 #include <cdio/util.h>
 #include <cdio/types.h>
 #include <cdio/mmc.h>
+#include <cdio/mmc_cmds.h>
 #include <cdio/audio.h>
 #include <cdio/cdtext.h>
 #include "cdtext_private.h"