* filesys.c (set_element): Return file descriptor.

* filesys.c (set_element): Return file descriptor.
(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.