* sysdeps/unix/opendir.c: If O_DIRECTORY_WORKS is defined don't
use the hack with tryopen_o_directory to determine at runtime
whether it works.
* sysdeps/unix/sysv/linux/opendir.c: New file.
* sysdeps/unix/sysv/linux/kernel-features.h: Define
__ASSUME_O_DIRECTORY if kernel knows O_DIRECTORY.