alcove_list_to_buf: include static binaries

Return information about the length of unallocated binaries so the
caller can return a list including the contents of the pointers, rather
than a binary including the pointers themselves.
alcove_buf_to_list: convert a buf to a cstruct

If an argument to prctl is a list, instead of returning a binary, return
a list. If the list contained allocated buffers, return the contents of
those buffers, rather than pointer.