add r_io_reopen

small test
``````
#include <r_io.h>
#include <stdio.h>

int main () {
	RIO *io = r_io_new ();
	RIODesc *desc = r_io_open_nomap (io, "malloc://32", R_IO_READ, 0644);
	r_io_desc_list (io);
	printf ("- reopen -\n");
	r_io_reopen (io, desc, R_IO_RW, 0644);
	r_io_desc_list (io);
	r_io_close (io, desc);
	r_io_free (io);
	return R_FALSE;
}
`````