This draft diff (please review) --
- moves ec184*, iskr103* and mc1502 out of pc.c
- moves CGA font upload support to a subclass
- adds new drivers: poisk1, ec1847, pk88
- adds a skeleton of native iskr103* keyboard
- adds more ROM revisions to existing drivers (how should I submit those?)
- etc.

- ec1841 can work with isa_hdc.c but not out of the box (WRITESBUFF command is not implemented and other quirks). Will submit a diff later. Native HDC boards are not dumped yet (one version used a Z80 + clone of some XEBEC chip, another a 8089 + 8253 + smaller chips).
- poisk1 is still WIP. Keyboard, video and floppy work but changing disk in drive A: or accessing drive B: causes I/O errors in DOS. Its HDC uses a WD1010 clone but our emulation of it is still WIP.