name array with array_<id> instead of struct_<id>

[ampotos]

When the option -k is used or when the analyze is run on a block via the web_ui all structure are named struc_ even if they look like an array.
Change the name of detected array by array_ will help to know this block is an array without having to go to the struct view.

That can be easyli done in Struct.clean_all_struct() (Struct.not_a_struct() return True when the block is detected as an array).