If ENABLE_CHECKING, range-check args of FD_CLR, FD_ISSET, FD_SET.

* process.c (add_read_fd, delete_read_fd, add_write_fd)
(delete_write_fd, wait_reading_process_output):
Remove now-redundant easserts.
* sysselect.h (SYSSELECT_H): New macro, to avoid double-inclusion woes.
Use INLINE_HEADER_BEGIN, INLINE_HEADER_END.
(fd_CLR, fd_ISSET, fd_SET): New inline functions.
(FD_CLR, FD_ISSET, FD_SET): Redefine in terms of these functions.