variable.c: Add _quiet versions of the var_set_ methods and use in var_clone

variable.c: Add _quiet versions of the var_set_ methods and use in var_clone

var_clone doesn't need to provoke callbacks, since the new variable does not
(yet) have a dictionary  hence the callbacks can do nothing.  Also the callbacks
cause problems for later commits, since they themselves could call var_clone.

This change therefore defines _quiet versions of all the var_set methods, which
do not generate any callbacks, and uses those versions in the implementation
of var_clone.