busybox

mount: add unc option to CIFS mount (needed for Linux 3.4+)

unc option for CIFS mount is mandatory after CIFS option parsing
was rewritten in Linux 3.4

Signed-off-by: Martin Santesson <martinsn@axis.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>