break out 1-1 SCSI-MMC commands (in mmc_ll_cmds.c) from higher-level commands which use the lower-level ones.
*mmc*: mmc_start_stop_media -> mmc_start_stop_unit, CDIO_MMC_GPCMD_START_STOP -> CDIO_MMC_GPCMD_START_STOP_UNIT cdio++/mmc.hpp: Regularize Doxygen comment format.