Windows: VolumeInfoReadOnlyFlag registry option

Windows 7 Explorer Shell has a bug when processing drive letters
mapped to UNC paths whose target volume information specifies the
FILE_READ_ONLY_VOLUME flag.  When set, not only is the .readonly
volume treated as read only but all volumes that can be accessed via the
drive letter.   This bug is fixed in Windows 8.

Add a registry configuration option to permit configuration of the
behavior.  Sites that do not use drive letter mappings will want to
enable it even on Windows 7 because it permits the Explorer Shell
to disable the "Delete" and "Rename" options and others when the current
directory is read only.

The default is disabled on Win7 and below; enabled on Win8 and above.

Change-Id: I73bbaf7d40918650b1a217ed44409c0679920536
Reviewed-on: http://gerrit.openafs.org/9606
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: Jeffrey Altman <jaltman@your-file-system.com>