Device configuration routine
int devswadd (dev_t devno, struct devsw *dswptr)
This adds a device entry pointed by
dswptr for a device specified by
devno to device switch table. It is typically called by the
ddconfig() routine of device driver.
int devswdel(dev_t devno)
This deletes a device driver entry from the device switch table. It is called by the
ddconfig() routine for termination of device driver.