Remove a few units from TD yard upgrades (mobile AA, nebel)

Limiting the flexibility of the TD upgrade should make for
a more important/defining choice than previously.

TD yard upgrade cost also dropped a bit: the low-los situation
at present makes TDs a little bit less overpowered than they
once were.  They're still more expensive, but less wildly so
(especially given their newfound weakness to air in the field).

Generally dropped TD upgrade cost/bt by 25%; SP went up by about
15%.