Define close_file slightly tighter:

       - It is permitted to call close_file with a zero filehandle
       - close_file zeros SI

Fix the handling of the new API in SYSLINUX.  It appears file_left was
never actually initialized; it simply didn't matter since the old code
would either trip on reaching the end of the FAT chain or count the
data it needed.  Now operating according to spec.

Fix the tsize error message to indicate that it now, in fact, refers
to an invalid TFTP response.

Support the new getfssec API which returns number of bytes read in
SYSLINUX and EXTLINUX.  Untested so far, but it's a reasonably easy
change.

The exact upper limit is 65536-256-2 = 65278 = 0xFEFE bytes.

The test for oversized COMBOOT files have to be updated to be an a
posteori test, since the length might not be known a priori.

gcc complains that fclose() is too big to be inlined with -Os, so
humour it and move fclose() out of line.

Remove obsolete and now unused filesize() function.

With loadfile() doing megabyte-sized trial-and-error allocations, we
want to be able to do really set the size on the final allocation;
thus constrain the amount of permitted slack to 4K max.

Handle unspecified file length for comboot/com32 modules; do changes
to the appropriate part of libcom32 to handle this properly.