MFC r232385 by ru: Remove 3 syscalls from opendir().

Finally removed the stat() and fstat() calls from the opendir() code.
They were made excessive in r205424 by opening with O_DIRECTORY.
Also eliminated the fcntl() call used to set FD_CLOEXEC by opening
with O_CLOEXEC.

(fdopendir() still checks that the passed descriptor is a directory,
and sets FD_CLOEXEC on it.)

The necessary kernel support for O_DIRECTORY and O_CLOEXEC was already in
9.0-RELEASE.

Discussed with:	ru