Dynamically allocate ata_channel info; introduce custom atadrive_s struct.

Dynamically allocate ata_channel info; introduce custom atadrive_s struct.

Don't limit the number of ATA controllers supported - just dynamically
allocate the structs.

Create an atadrive_s struct that extends the standard 'struct drive_s'
and have the new struct store a pointer to the ata channel info.

Also, prefer storing drive_s pointers as 32bit "flat" pointers -
adjust them as needed in the 16bit code.