heath/h17_fdc.cpp: Skeleton impl for the H-88-1 Hard-Sectored Controller

[mgarlanger]

Initial code for the Heath H-88-1 Hard-sectored controller for the H-89 family of systems. This supported up to 3 floppy drives.

This initial code implements the I/O ports of the controller, includes the AMI S2350 USRT, the transmitter clock for the USRT.

Things still needed

• MAME's core code must support hard-sectored disk images.
• Receiver clock for the USRT, based on read clock bits from the disk.
• Add support for a Heath hard-sectored disk image format.
• Add controller to the H89 systems.