Basic c++ification of varobj

Basic c++ification of varobj

This patch does a basic c++ification or the varobj data structure.

  - varobj: add constructor and destructor, initialize fields
  - varobj_root: initialize fields
  - varobj_dynamic: initialize fields

This allows getting rid of new_variable, new_root_variable.
free_variable essentially becomes varobj's destructor.  This also allows
getting rid of a cleanup, make_cleanup_free_variable, which was only
used in varobj_create in case the varobj creation fails.  It is replaced
with a unique_ptr.

gdb/ChangeLog:

	* varobj.h (struct varobj): Add constructor and destructor,
	initialize fields.
	* varobj.c (struct varobj_root): Initialize fields.
	(struct varobj_dynamic): Initialize fields.
	(varobj_create): Use unique_ptr instead of cleanup.  Create
	varobj with new instead of new_root_variable.
	(delete_variable_1): Free variable with delete instead of
	free_variable.
	(create_child_with_value): Create variable with new instead of
	new_variable.
	(varobj::varobj): New.
	(varobj::~varobj): New (body mostly coming from free_variable).
	(new_variable): Remove.
	(free_variable): Remove.
	(do_free_variable_cleanup): Remove.
	(make_cleanup_free_variable): Remove.