Added the following new machines:

- Olivetti M19.
- Olivetti M240.

Implemented mainboard jumper settings for Olivetti M24 machine:
- Like on a real M24, onboard video card can be disabled, and other video cards can be installed.
- Renamed m_olivetti_m24.c to m_xt_olivetti.c, as it handles all XT-class Olivetti machines.

Decoupled Olivetti OGC (640x400 extended CGA) from M24 machine code:
- OGC exists both as 16-bit proprietary card and 8-bit ISA card (GO708).
- Implemented underlined text handling in 80x25 character mode.
- Like Compaq CGA, replaced code portions common to CGA with calls to vid_cga.c.

src/include/86box/machine.h

@@ -509,8 +509,10 @@ extern int	machine_europc_init(const machine_t *);
 extern const device_t europc_device;
 #endif
 
-/* m_oivetti_m24.c */
-extern int	machine_olim24_init(const machine_t *);
+/* m_xt_olivetti.c */
+extern int	machine_xt_olim24_init(const machine_t *);
+extern int	machine_xt_olim240_init(const machine_t *);
+extern int	machine_xt_olim19_init(const machine_t *);
 #ifdef EMU_DEVICE_H
 extern const 	device_t *m24_get_device(void);
 #endif