dont delete a side from carryover if it came back to life

the previous behaviour was inconsitent, since [team]lost= wasn't read when loading a savegame. So this doesn't break previous behaviour.

This especialy fixes the scenario "Stolen Gold" from "Northern Rebirth" (http://gna.org/bugs/?22601)
read lost= from [team] when loading a savefile
Merge pull request #281 from wesnoth/carryover_fix_1_12

don't remove a side from carryover if it came back to life