Use varobj_is_dynamic_p more widely

Use varobj_is_dynamic_p more widely so that the callers of
varobj_is_dynamic_p are unchanged when we add available-children-only
stuff in varobj_is_dynamic_p.

gdb:

2014-06-12  Yao Qi  <yao@codesourcery.com>

	* varobj.c (varobj_get_num_children): Call
	varobj_is_dynamic_p.
	(varobj_list_children): Likewise.
	(varobj_update): Likewise.  Update comments.