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.

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>
(cherry picked from commit f8f8c0a48fbe086870a0cf35a62278c2b4ab743b)

Change-Id: If19cafed9d19ce5f6a529c64fe48a03c13cb0e03
Reviewed-on: http://gerrit.openafs.org/3784
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>