Clear any mouse hot-spots before executing external program from exec_xtrn()

As Nelgin pointed out, mouse hot-spots were still active when running external
programs (door games) - so clear any active hot-spots before running external
programs.

This could possibly be done in external() instead, but this sufficient for
now. I wouldn't want a menu that EXEC's something (via @-code) to lose all its
hot-spots as a result.