more melee for aliens

units may now have a built in melee weapon that doesn't occupy a slot,
as defined in the unit section of the ruleset under the tag "meleeWeapon:"
with an accompanying string referring to an item.

this is only available to the AI, and as such some adjustments have taken
place to make aliens decide which to use, given their current situation,
as per original TFTD.

units can have a list of items that are "built in", and these items will
be given to the unit automatically. additionally, fixed weapons and alien
weapons can now be allocated to the left hand, meaning HWPs can have
secondary weapon systems installed.
whoops
arcing projectiles

curvature should start at (1.73 / sqrt(sqrt(70.0 / 10.0)) = 1.06358350461)
rather than 0.5 for arcing shots, as strength and weight are assumed to be
70 and 10 respectively
smoke inhalation by ruleset
haha derp
fix up occasional weird error message with TU reserve
rethink: tie error message to setting

the underlying logic can stay the same, but the error message should
probably be consistent with the player's reservation setting.
Merge branch 'master' of https://github.com/SupSuper/OpenXcom