Reset oo_.{exo_}steady_state to zero before endval

This is necessary to enforce the documented behavior which says that, if a
variable is not given a value in the endval block, a zero value is assumed.
(cherry picked from commit cf146ec4e50b83bb17f6890b936d8d89e7ef6664)
Fix handling of auxiliary variables in endval block
(cherry picked from commit a7e29af81c77df2a72a0dc3fa08e4730f08bc891)