vol: namei_ops improve readability; fix namei_create on Windows

vol: namei_ops improve readability; fix namei_create on Windows

use NAMEI_SPECDIRC, NAMEI_INODESPECIAL, INVALID_FD and other
preprocessor symbols in place of numeric constants.

namei_create() on Windows must call namei_CreateDataDirectories()
and retry if the afs_open() call fails with either ENOTDIR or
ENOENT.

modify namei_HandleToName() on Windows to make its decision
process clearer.

Change-Id: Ib1dfbaa3afe81f57685c95cdcae749c67cc95598
Reviewed-on: http://gerrit.openafs.org/3712
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>