refactor: eliminate the getunitpeasants global variable.
bugfix: it was possible to uncover stealth units with GIVE CONTROL.
remove the deprecated getunit function.
fix the build, missing return value from give_control_cmd.
add the test coverage for getunit.
fix a bug where NULL was not returned when unit is in the wrong region.
Merge pull request #66 from badgerman/getunit-refactor

refactoring getunit