Feature: Separate rail/road and sea/air velocity units, and add knots. (#10594)

This is achieved by packing vehicle type along with the velocity so that
the string system can decode and pick the appropriate unit.

src/table/settings/locale_settings.ini

@@ -73,7 +73,7 @@ type     = SLE_UINT8
 from     = SLV_184
 flags    = SF_NO_NETWORK_SYNC | SF_GUI_DROPDOWN
 def      = 1
-max      = 3
+max      = 4
 full     = _locale_units
 post_cb  = [](auto) { MarkWholeScreenDirty(); }
 cat      = SC_BASIC
@@ -81,6 +81,20 @@ str      = STR_CONFIG_SETTING_LOCALISATION_UNITS_VELOCITY
 strhelp  = STR_CONFIG_SETTING_LOCALISATION_UNITS_VELOCITY_HELPTEXT
 strval   = STR_CONFIG_SETTING_LOCALISATION_UNITS_VELOCITY_IMPERIAL
 
+[SDT_OMANY]
+var      = locale.units_velocity_nautical
+type     = SLE_UINT8
+from     = SLV_VELOCITY_NAUTICAL
+flags    = SF_NO_NETWORK_SYNC | SF_GUI_DROPDOWN
+def      = 1
+max      = 4
+full     = _locale_units
+post_cb  = [](auto) { MarkWholeScreenDirty(); }
+cat      = SC_BASIC
+str      = STR_CONFIG_SETTING_LOCALISATION_UNITS_VELOCITY_NAUTICAL
+strhelp  = STR_CONFIG_SETTING_LOCALISATION_UNITS_VELOCITY_HELPTEXT
+strval   = STR_CONFIG_SETTING_LOCALISATION_UNITS_VELOCITY_IMPERIAL
+
 [SDT_OMANY]
 var      = locale.units_power
 type     = SLE_UINT8