rpcclient: append a trailing slash to FSRVP request UNCs

rpcclient: append a trailing slash to FSRVP request UNCs

The Windows Server 2012 FSRVP server exhibits strange behaviour when
exposing hidden shadow copy shares. If the hidden share UNC in the
AddToShadowCopySet request includes a trailing backslash (e.g.
"\\server\share$\"), then the new shadow-copy share will also be hidden
(e.g. "\\server\share$@{ShadowCopy.ShadowCopyId}$").
However, if the UNC does not include a trailing backslash, which until
now was rpcclient's default behaviour, then the exposed shadow-copy
share is not hidden.

Thanks to the MS Open Specifications team for helping me track down this
one.

bug: https://bugzilla.samba.org/show_bug.cgi?id=10521

Signed-off-by: David Disseldorp <ddiss@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>