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/video/vid_table.c

@@ -177,6 +177,7 @@ video_cards[] = {
     { "virge375_vbe20_vlb",	&s3_virge_375_4_vlb_device		},
     { "tgui9400cxi_vlb",	&tgui9400cxi_device			},
     { "tgui9440_vlb",		&tgui9440_vlb_device			},
+    { "ogc",        		&ogc_device				},
     { "",			NULL                        		}
 };