Check to see if already included.

Check to see if already included.
(fd_set, filedesc): New structures.
(child_process, MAX_CHILDREN, CHILD_ACTIVE): Definitions moved
from ntproc.c.
(FD_SET, FD_CLR, FD_ISSET, FD_ZERO): Operate on fd_set structures.
(SELECT_TYPE): New macro.
New child process status enumeration.
(FILE_READ, FILE_WRITE, FILE_BINARY, FILE_PIPE, FILE_SOCKET):
New macros.
(fd_info, new_child, delete_child): Declared.