Windows: afs_shl_ext folder bkgrnd context menu

Windows: afs_shl_ext folder bkgrnd context menu

Make the context menu handler also work for folder backgrounds
and on Win7 for library folder backgrounds

For folder backgrounds, the shell passes the PIDL of the folder
instead of a data object.  Extract the path from that PIDL. Also
extended the register function of the dll to add the required
registry keys.

Change-Id: I8928efd25058dced3820478a2858ce20336b4301
Reviewed-on: http://gerrit.openafs.org/3443
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Stefan Kueng <tortoisesvn@gmail.com>
Reviewed-by: Stefan Kueng <tortoisesvn@gmail.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>