Ruleset adjustments

Ruleset: remove arbitrary modIndex increment
RuleItem: add safety exit if surface invalid
Inventory: add safety exits if surface invalid
XcomResourcePack: LOG_INFO for extraSprites
	- remove TIFF from valid image extensions
NewBattleState: switch some buttons
BattlescapeGenerator: cleanup deployAliens()
Xcom1Ruleset: remove -countries, let ImprovedNations handle it
	- use custom bigobs (bigSprites) for CivM & CivF
	- alter string "to assault" to "ready to assault"
XCOM1_Improved_Nations: re-instantiate Nigeria
	- plus a couple of country-name changes
	- comments
UnitSprites: externalize BIGOBS.PCK
	- and add CivM & CivF corpse images