#15 Add transport-only reinforcement type for existing units

[Z Beeblebrox]

Infantry units at the start of many scenarios do not have transport. If player has "prestige points", then he should be able to buy transport for units that do not have it.
Obviously, price of transport upgrade will depend per unit type, but currently there is no alternative other than to split a unit already having transport in half, then combine with a split unit that has no transport. This makes no sense, as a player with prestige should be able to send transport to infantry unit on foot.