(fill_select_type): Return the highest file descriptor.
(scm_select): Tell select about the highest file descriptor. On
some systems the SELECT_SET_SIZE can be as much as 128 bytes.
Therefore the extra overhead for calculating the maximum fd seems
to be more than compensated. Is this correct? In any case,
scm_internal_select will be much faster with this info.
(scm_select, fill_select_type, set_element): Don't accept any kind
of object in the file descriptor list or vector.
coop_wait_for_runnable_thread): Disable interrupts so that no
async is executed before a potential error_revive.
(scm_internal_select): Disable interrupts during the parts of the
code which manipulate the sleep queue and the file descriptors.
