Make Wall its own TargetType

Going to make some units (machineguns, etc.) unable to fire at walls.
Certain units can no longer target walls

Units which cannot target walls (sandbags, chain-link fence or concrete) any longer:
Minigunner
Flamethrower
Hum-vee/Nod Buggy 
Flame Tank (can still run over sand bags and chain-link fences)
Civilian
[sniper--already so]
Merge pull request #5194 from psydev/patch-8

Certain units can no longer target walls