Added two baud rates (28.8k and 33.6k)

According to Windows 98 modem setup.

src/network/net_modem.c

@@ -1566,6 +1566,8 @@ static const device_config_t modem_config[] = {
             { .description =  "57600", .value =  57600 },
             { .description =  "56000", .value =  56000 },
             { .description =  "38400", .value =  38400 },
+            { .description =  "33600", .value =  33600 },
+            { .description =  "28800", .value =  28800 },
             { .description =  "19200", .value =  19200 },
             { .description =  "14400", .value =  14400 },
             { .description =   "9600", .value =   9600 },