Windows: Make default mode bits configurable

Windows: Make default mode bits configurable

Mode bits aren't directly exposed by the Win32 API. We were leaving
them to default to 0777 when creating new files and directories.

This patch introduces two configuration parameters;
'UnixModeFileDefault' and 'UnixModeDirDefault' which are DWORD
registry entries that are used to set the initial mode bits.

If the values are set to 0, then the behavior is identical to what we
had before.

Change-Id: I7822141aab1e12cc8e151be771f779284fa68465
Reviewed-on: http://gerrit.openafs.org/1602
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Asanka Herath <asanka@secure-endpoints.com>
Tested-by: Asanka Herath <asanka@secure-endpoints.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>