Added support for SCSI_CMD_PREVENT_ALLOW_MEDIUM_REMOVAL

[acscd]

This adds support for SCSI_CMD_PREVENT_ALLOW_MEDIUM_REMOVAL via a callback function tud_msc_prevent_allow_medium_removal_cb, very similar to the handling of SCSI_CMD_START_STOP_UNIT.

Properly handling the Prevent / Allow Medium Removal command clears up a "Failed to eject device" error on Linux.