This function closes a file identifed by a file descriptor, ie. that was opened by fd_open.
Gamma> require_lisp("const/filesys");
"const/filesys"
Gamma> fp = fd_open("/fd/ttyp8",O_WRONLY);
4
Gamma> fd_write(fp,"\nHello\n");
8
Gamma> fd_close(fp);
t
Gamma> fd_close(fp);
nil