struct dirent *, rather than non-const. this makes scandir(3) the
same as the scandir implementations in libiberty and glibc, and the
select function has no need to modify the dirent.
as __null with egcs 1.0 (GCC 2.90) and above. As several headers are affected
by this change, move the definition into a new header file, <null.h>, to ease
maintenance.
Some programs (like NSCA httpd) include <dirent.h> without first including
<sys/types.h> so the u_foo types aren't declared. I don't think POSIX.1
allows us to include <sys/types.h> in <dirent.h> either.
