Main recio: Allow rec_io_(start|stop)_read from the read_handler

Main recio: Allow rec_io_(start|stop)_read from the read_handler

This involves rescanning the read buffer on rec_io_start_read()
as there might be leftover data from previous invokations of the
read_handler.

However, we have to be careful to avoid arbitrarily deep recursion
of various handlers. We do that by deferring the actual start of
reading to a main_hook.